U.S. patent application number 13/696380 was filed with the patent office on 2013-05-09 for devices and methods for testing analytes.
This patent application is currently assigned to Exacsys Limited. The applicant listed for this patent is David Edington, Mark Humphries, Carys Lloyd, Craig Nelson, Michael Noble, John Rippeth. Invention is credited to David Edington, Mark Humphries, Carys Lloyd, Craig Nelson, Michael Noble, John Rippeth.
Application Number | 20130112573 13/696380 |
Document ID | / |
Family ID | 42315024 |
Filed Date | 2013-05-09 |
United States Patent
Application |
20130112573 |
Kind Code |
A1 |
Noble; Michael ; et
al. |
May 9, 2013 |
DEVICES AND METHODS FOR TESTING ANALYTES
Abstract
A method and device are provided for measuring a level of a
clinically relevant analyte (such as glucose) in a fluid (such as
blood). The device includes a flow path for conducting said fluid
through the device; a detection chamber arranged on said flow path;
and detector means arranged to detect analyte levels in the fluid
in said chamber, wherein: said detection chamber contains a
predetermined amount of an analyte such that that analyte mixes
with fluid in the detection chamber to form, at the detector means,
a calibration sample of the fluid at a time after the arrival of
the fluid in said detection chamber, and said detector means is
arranged to detect a first analyte level of an unadulterated sample
of the fluid at a first time which is before the formation of said
calibration sample and to detect a second analyte level of said
calibration sample at a second time which is after the formation of
said calibration sample.
Inventors: |
Noble; Michael; (Melbourn,
GB) ; Nelson; Craig; (Melbourn, GB) ;
Humphries; Mark; (Melbourn, GB) ; Lloyd; Carys;
(Melbourn, GB) ; Edington; David; (Melbourn,
GB) ; Rippeth; John; (Melbourn, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Noble; Michael
Nelson; Craig
Humphries; Mark
Lloyd; Carys
Edington; David
Rippeth; John |
Melbourn
Melbourn
Melbourn
Melbourn
Melbourn
Melbourn |
|
GB
GB
GB
GB
GB
GB |
|
|
Assignee: |
Exacsys Limited
London
GB
|
Family ID: |
42315024 |
Appl. No.: |
13/696380 |
Filed: |
May 6, 2011 |
PCT Filed: |
May 6, 2011 |
PCT NO: |
PCT/GB11/00703 |
371 Date: |
January 11, 2013 |
Current U.S.
Class: |
205/777.5 ;
204/403.14 |
Current CPC
Class: |
G01N 27/3272 20130101;
G01N 33/4915 20130101; C12Q 1/54 20130101; G01N 2496/00 20130101;
C12Q 1/006 20130101; G01N 27/3274 20130101; G01N 33/49 20130101;
G01N 27/3271 20130101 |
Class at
Publication: |
205/777.5 ;
204/403.14 |
International
Class: |
C12Q 1/00 20060101
C12Q001/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 7, 2010 |
GB |
1007711.3 |
Claims
1.-50. (canceled)
51. A device for measuring a level of a clinically relevant analyte
in a fluid, including: a flow path for conducting said fluid
through the device; a detection chamber arranged on said flow path;
and detector means arranged to detect analyte levels in the fluid
in said chamber, wherein: said detection chamber contains a
predetermined amount of an analyte such that that analyte mixes
with fluid in the detection chamber to form, at the detector means,
a calibration sample of the fluid at a time after the arrival of
the fluid in said detection chamber, and said detector means is
arranged to detect a first analyte level of an unadulterated sample
of the fluid at a first time which is before the formation of said
calibration sample and to detect a second analyte level of said
calibration sample at a second time which is after the formation of
said calibration sample.
52. A device according to claim 51 wherein the analyte in said
predetermined amount is the same as said clinically relevant
analyte.
53. A device according to claim 51 wherein said device has a single
detector means.
54. A device according to claim 51 wherein said device has a single
flow path.
55. A device according to claim 51 wherein said predetermined
amount of analyte is located at a position within the detection
chamber remote from the position of the detector means.
56. A device according to claim 51 wherein said predetermined
amount of analyte is located on a substrate at a predetermined
position in relation to the position of the detector means.
57. A device according to claim 56 wherein the flow path and
detection chamber are formed by a plurality of structural
components and said substrate forms part of at least one of said
structural components.
58. A device according to claim 56 where the predetermined location
of the analyte is within an area on said substrate which has a
different surface energy to other areas on the same substrate, and
the substrate on which the predetermined amount of analyte is
located is generally hydrophobic and the formulation containing the
predetermined amount of analyte provides a surface which is more
hydrophilic than the substrate and which facilitates capillary fill
of the flow path and detection chamber.
59. A device according to claim 56 where the predetermined location
of the analyte is within an area on said substrate which has a
different surface energy to other areas on the same substrate, and
wherein the substrate on which the predetermined amount of analyte
is located is generally hydrophobic and is covered by a layer of
hydrophilic material which forms a wall of the flow path.
60. A device according to claim 51 wherein the predetermined amount
of analyte is contained within a formulation containing a
thickening agent.
61. A device according to claim 51 wherein the predetermined amount
of analyte is positioned in a line across the width of the flow
path or detection chamber in a direction orthogonal to the
direction of flow through the flow path or into the detection
chamber.
62. A device according to claim 51 wherein the detector includes a
working electrode and said predetermined amount of analyte and the
working electrode of the detector are located on opposing surfaces
of the detection chamber.
63. A device according to claim 62 wherein at least part of said
predetermined amount of analyte is located directly opposite at
least part of said working electrode.
64. A device for measuring clinically relevant analyte according to
claim 51 wherein the device includes at least 2 detector means,
wherein a first detector means is optimised for measuring the
clinically relevant analyte, and a second detector means is
optimised for measuring a non-analyte-dependent signal, such that
the non-analyte-dependent signal can be used to obtain information
about the analyte level measured by the first of said detector
means.
65. A device according to claim 64 wherein the second detector
means is arranged to detect a quantity related to the diffusivity
of the clinically relevant analyte.
66. A device according to claim 65 wherein the first detector means
includes a redox mediator and the second detector means does not
include said redox mediator, and wherein the detector means are
arranged such that the measurement of the non-analyte-dependent
signal is made before said redox mediator is able to diffuse to the
second detector means.
67. A method for testing, in a portable device, a level of a
clinically relevant analyte in a fluid, the method including the
steps of: guiding a sample of the fluid to a detection chamber
containing a known amount of an analyte; measuring an analyte level
in an unadulterated sample of the fluid at a first predetermined
time after the arrival of the fluid in said detection chamber;
measuring an analyte level in a calibration sample of the fluid
which has mixed with said known amount of said analyte at a second
predetermined time after the arrival of the fluid in said detection
chamber, being later than said first predetermined time; and
adjusting the analyte level measured in said unadulterated sample
using the analyte level measured in said calibration sample.
68. A method according to claim 67 wherein said analyte in said
known amount of analyte is the same as said clinically relevant
analyte.
69. A method according to claim 67 wherein the step of adjusting
uses the rate of diffusion or the time taken to detect the effects
of the mixing of the predetermined amount of analyte, or a
combination thereof, to adjust the analyte level measured in said
unadulterated sample.
70. A method according to claim 67 further including the steps of:
storing a record of said non-analyte specific information; updating
said record after subsequent measurements; and determining a
deviation from said record in a subsequent measurement; and,
optionally, if said deviation is greater than a predetermined
amount, alerting the user.
Description
[0001] The present invention relates to devices and methods for
testing clinically relevant analytes in samples of biological
origin. It is particularly, but not exclusively concerned with
methods and devices with improved accuracy for testing blood
glucose levels.
[0002] Examples of clinically relevant analytes are indicators of
cardiovascular, liver and kidney function (e.g. cholesterol,
haemoglobin, electrolytes, metabolites), infectious agents (e.g.
viruses, bacteria), disease state indicators (e.g. C-reactive
protein, antibodies, cellular signalling factors, hormones) and
therapeutic agents (e.g drugs) and in particular glucose. Examples
of samples of biological origin include cerebrospinal fluid, urine,
semen, saliva and in particular whole and fractionated blood.
[0003] In recent years, there has been growth in the number and
types of clinical diagnostic tests carried out outside of the
traditional laboratory setting. Many test systems have been
developed and marketed for use at the "Point of Care" ("POC"),
including systems which are used at hospital bedsides, in intensive
care units, in the offices of doctors or physicians and in the
patient's home or at other locations which are desirable or
convenient.
[0004] In some instances these tests may be carried out by
healthcare professionals (although not necessarily trained
laboratory staff) but in others the patients themselves conduct the
testing to help monitor and manage their own condition. The most
common self testing systems are for blood glucose monitoring
("BGM") in patients with diabetes, but other tests, such as blood
coagulation time testing for patients undergoing Warfarin therapy,
are also becoming more common.
[0005] In particular the management of diabetes has been
considerably improved by the availability and use of self-testing
Blood Glucose Monitoring (BGM) systems. These systems allow
diabetics to determine their own blood glucose levels and,
depending on the result, to adjust their therapy or modify their
diet, thus maintaining their glucose levels within a relatively
narrow band within what is considered to be a healthy level.
[0006] This ability for diabetics to test and consequently control
their own blood glucose levels tends to result in a lower incidence
of diabetes related complications, as evidenced by the 1993
Diabetes Control and Complications Trial (DCCT, New England Journal
of Medicine 329 (14), Sep. 30, 1993).
[0007] In addition to the difference in the training and experience
of the person conducting clinical diagnostic tests from a trained
laboratory technician to a more generalised healthcare practitioner
or patient, there are a number of other key differences between POC
tests and lab-based tests. These include the relatively
uncontrolled (and therefore variable) testing conditions in the POC
setting (e.g. ambient temperature and humidity), a lower level of
control of the storage conditions and consequential ageing of
diagnostic chemistries and variations in the biological sample
matrix for the analyte under test. Mass produced POC test systems
generally do not take these variabilities into account when
interpreting readings and providing results.
[0008] Moreover, the very high number of tests that can be carried
out in the POC setting (several billion BGM test strips are used
every year) means that manufacturing methods have to be very well
controlled to maintain consistency of readings and results both
within and between batches.
[0009] Each of these factors can individually cause error in the
diagnostic performance of a POC test system, thereby undermining
the advantages that this mode of testing can offer.
[0010] For example, the accuracy of the blood glucose readings
generated by existing BGM systems (which typically use an enzyme
electrode where an enzyme such as glucose oxidase or glucose
dehydrogenase catalyses the reaction of glucose producing an
electrically measurable signal, potentially via use of an
electrochemical redox mediator) can be adversely affected by a
number of factors, such as: [0011] 1. Variability in hematocrit
level of test blood. Since BGMs are generally designed to function
in a diffusion limited manner, where the signal is dependent on the
flux of glucose molecules to the sensor surface which is in turn
related to the bulk concentration, then variability in the
diffusive properties of the sample matrix can lead to testing
inaccuracy. In blood, this effect is most apparent with variable
hematocrit levels, where, for instance, at higher than normal
levels the presence of a greater concentration of red blood cells
can impede diffusion, slowing flux of the analyte to the electrode
surface. [0012] 2. Variation in the prevailing environmental
conditions during testing. BGM systems are sensitive to the
temperature at which testing is being conducted since temperature
affects both enzyme kinetics and diffusion rates. Additionally,
altitude can have an impact since it may affect the oxygen
availability at the electrode surface--oxygen is required in
GOD-catalysed reactions in certain electrode configurations, but
can also impede the overall reaction rate by competing with redox
mediators in alternative configurations. [0013] 3. Instability of
the reaction components--particularly the enzymes used in the
detectors which may be subject to a change of activity due to
temperature and/or humidity effects over the storage time.
Electrochemical redox mediators used in some systems can have
similar environmental sensitivity thus providing a further
contribution to error from an environmental source. [0014] 4. Lack
of specificity of the enzyme--whilst GOD is a highly specific
enzyme with limited capability to catalyse the oxidation of
molecular species other than glucose, enzyme electrodes based on
other enzymes can be less specific. For example, certain glucose
dehydrogenase (GDH) based systems with the redox mediator
pyrroloquinoline quinine (PQQ) are known to cross react with other
sugars such as maltose, galactose and xylose and since these
substances may be found in certain drug and biologic formulations,
this can lead to a falsely high `glucose` reading. [0015] 5. The
presence of interfering substances in the patient's blood--some
substances such as acetaminophen and ascorbate are electroactive
and can themselves generate an electrical signal on being oxidised
at an electrode surface [0016] 6. Manufacturing process
variation--strips are manufactured in large batches and a
calibration is determined for each batch to enable conversion of
the electrical signal into a blood glucose reading. The calibration
set for a particular batch reflects the average performance of the
strips in terms of their electrical response to blood glucose over
the range of clinically feasible values. However, for any one
particular strip, it is likely that it will deviate from the
average and so the `batch calibration` applied to the signal
generated by that strip will not provide an accurate reading of the
blood glucose concentration in the sample. [0017] 7. Inappropriate
calibration--strip batches are typically calibrated for their
response to blood glucose and each manufactured batch has a
`calibration code` that enables the meter to convert
electrochemical readings into glucose concentrations. Because there
is variation between batches, calibration codes may differ.
Inaccuracy may therefore arise if a meter is using a calibration
code that is inappropriate for the batch of strips being used in
testing. [0018] 8. Improper testing procedure--modern BGM systems
are designed to reduce the likelihood of improper testing
procedure, or `user error`. However, there is a possibility that
BGM systems will be used outside of their specified conditions of,
for instance, temperature or altitude. Equally, users might
inadvertently affect the accuracy of their blood glucose readings
by improper preparation of the blood test site, for example by
insufficient drying of wet hands leading to a diluted blood
sample.
[0019] Whilst these error sources have been described with
electrochemical detection of enzyme generated electroactive
species, similar errors can occur with optical and spectral
detection systems.
[0020] As a result of these and other sources of error,
commercially available BGM systems typically have system accuracy
levels (also known as total error) of up to 20%, compared to a
reference laboratory method.
[0021] Over the years since introduction of self monitoring of
blood glucose there have been calls for tighter accuracy standards
from bodies such as the American Diabetic Association (ADA). For
example in 1993 a consensus statement was issued recommending an
accuracy target of .+-.5% for BGM systems. More recently, the FDA
has indicated that it is unhappy with the current .+-.20% accuracy
standard (for 95% of readings) and has stated that it may consider
recognising a higher performance standard in future.
[0022] Examples of known BGM devices and disadvantages associated
with them are discussed in the introductory portion of
WO2005/080970, the contents of which are hereby incorporated by
reference.
[0023] Whilst the above discussion has focussed primarily on BGM
systems, similar problems are experienced with devices and methods
for measuring other clinically relevant analytes.
[0024] In order to improve system accuracy, current approaches are
focussed on the recognised sources of error in POC tests.
[0025] For example, error arising from manufacturing imprecision
can be addressed by improving the manufacturing process control
and, potentially, investing in new manufacturing technologies such
as advanced printing or laser ablation techniques.
[0026] Attempts to minimise the adverse impact of variable
environmental conditions such as temperature and altitude are
focussed on developing sensing and signal processing mechanisms
that modify the raw signal according to an algorithm.
[0027] In terms of the presence of interferents, effort is
concentrated on development of new chemistries or sensing
technologies that are less sensitive to these interferences or the
use of selective membranes that can specifically exclude certain
interferents from the sensing electrode. Sample effects, such as
variable hematocrit, are often corrected for by electrochemical
systems that measure the extent of these effects and make a
correction accordingly.
[0028] Finally, attempts to reduce the likelihood of user error
include the development of systems where the user intervention is
more controlled, for instance by controlling blood introduction
into the strip by the use of a capillary fill method.
[0029] Each of these approaches requires significant investment in
terms of time and money. Moreover, the system accuracy improvement
from these approaches can be limited, since each approach typically
addresses only a limited number of sources of error.
[0030] AgaMatrix has developed a method (described in
US2009/166225) of correcting an analyte-dependent signal by using
an analyte-independent signal. This method may require complex
sensors and associated signal processing capabilities and require
longer test times to allow for the collection of sufficient data
features to enable a correction to be made.
[0031] Nova Biomedical has developed a BGM system, the Stat Strip,
which utilises a series of measurement wells to measure
electroactive interferences and haematocrit independently of the
analyte glucose (U.S. Pat. No. 6,287,451). This information is used
to correct the measured glucose signal and thereby aims to provide
a more accurate reading.
[0032] Such methods of measuring an analyte independent signal may
have relatively low sensitivity because the analyte independent
signal and the analyte dependent signal are measured under the same
test conditions, which are usually optimised for the analyte
dependent signal measurement. Accordingly, the low sensitivity of
analyte independent signal measurements affords only limited
ability to correct the analyte dependent signal.
[0033] Despite the application of many methods to reduce BGM system
error, there still remains significant inaccuracy in BGM systems
marketed today. Error caused by variability in the hematocrit level
of the test sample and the test temperature are currently amongst
the most significant remaining sources that have not been
adequately addressed.
[0034] WO2005/080970, referred to above, discloses a method for
improving the accuracy of POC test devices (including BGM devices)
by adding a predetermined amount of analyte to be tested to the
test sample and comparing the resultant signal with that from an
unadulterated sample. This method enables a correction to be made
to the unadulterated sample reading based on the readings in the
sample(s) with analyte added. In effect this enables an element of
`live`, (or `on strip`) calibration and aims to minimise or
eliminate testing error from resulting from many error sources
including enzyme electrode instability, sample effects (such as
haematocrit level), improper test procedure (such as inappropriate
temperature), inappropriate calibration code and manufacturing
imprecision.
[0035] The devices and methods set out in WO2005/080970 generally
rely on calibration by spatial separation of the unadulterated and
calibration samples (either to use separate detectors or separate
flow paths to a common detector).
[0036] The present invention seeks to provide devices and methods
which have improved accuracy and remain simple and cheap to
manufacture.
[0037] It is a further object of the present invention to provide
methods of calibration which are compatible with existing devices
without significant alteration of those devices, and devices which
can be manufactured using essentially existing production
processes.
[0038] At its broadest the present invention provides methods and
devices for measuring levels of clinically relevant analytes, such
as glucose, in a fluid and calibrating that measurement as a result
of measurement of a calibration sample within the same device.
[0039] At its broadest a first aspect of the present invention
provides a method for measuring levels of clinically relevant
analytes, such as glucose, in a fluid and calibrating that
measurement as a result of subsequent measurement of a calibration
sample by the same detector.
[0040] A first aspect of the present invention provides a method
for testing, in a portable device, a level of a clinically relevant
analyte in a fluid, the method including the steps of: guiding a
sample of the fluid to a detection chamber containing a
predetermined amount of an analyte; measuring an analyte level in
an unadulterated sample of the fluid at a first predetermined time
after the arrival of the fluid in said detection chamber; measuring
an analyte level in a calibration sample of the fluid which has
mixed (either completely or in part) with said predetermined amount
of said analyte at a second predetermined time after the arrival of
the fluid in said detection chamber, being later than said first
predetermined time; and adjusting the analyte level measured in
said unadulterated sample using the analyte level measured in said
calibration sample.
[0041] Any of the following optional features (including
combinations of some or all such features) may be included in the
method of this first aspect.
[0042] The detection chamber may form part of the flow path or be a
separate chamber connected to the flow path.
[0043] In one embodiment said analyte in said predetermined amount
of analyte is the same as said clinically relevant analyte. As a
result the measurement means both for the unadulterated sample and
the calibration sample can be the same, and so errors that affect
the measurement means can be detected and adjusted for.
[0044] As both the unadulterated sample and the calibration sample
are present or formed in the detection chamber, they can both be
measured by a single detector. Thus there is a temporal separation
between the measurement of the unadulterated sample and the
measurement of the calibration sample and any errors due to
variation in the detectors used for measuring the levels in the
calibration and unadulterated samples can be avoided.
[0045] Furthermore, as both the unadulterated sample and the
calibration sample are formed from the same volume of fluid, there
is no requirement for an additional amount of fluid to be provided
for the analysis.
[0046] In alternative embodiments, the calibration analyte may be a
different chemical species to the clinically relevant (target)
analyte. In one such arrangement the calibration analyte is
measurable by part of the overall measurement means used for the
target analyte. For example, the calibration analyte might be
detected directly at the electrode by electrochemical oxidation
under the same, or different, conditions as those used for the
target analyte. This arrangement could enable inferences to be made
about sub-components of the target analyte measurement (for
example, the test temperature affecting the calibration analyte
diffusion rate and the electrode dimensions), and allow corrections
to the target analyte concentration accordingly. In certain
configurations, the use of a calibration analyte of a different
chemical species may be advantageous as the different species will
not interfere with the target analyte measurement, or will
interfere to a lesser extent than if the clinically relevant
analyte is also used as the calibration analyte. Alternatively, the
calibration analyte could have a particular sensitivity to a
component or variable of the test sample that is known to cause
erroneous readings.
[0047] In certain embodiments, measurement of the calibration
sample formed is substantially insensitive to the amount of said
predetermined analyte which is present. For example, the step of
adjusting may use the rate of diffusion or the time taken to detect
the effects of the mixing of the predetermined amount of analyte,
or a combination thereof, to adjust the analyte level measured in
said unadulterated sample.
[0048] In further alternative embodiments, the analyte and
calibration measurements may be carried out under different
regimes. In one such arrangement there may be a separate detector
means where the second (or further) calibration measurements are
optimised for measuring an analyte independent signal, in contrast
to the primary detector which is optimised for measuring the level
of the target analyte. For example the separate detector means may
be overlaid with the same reagent(s) as used for the primary
detector means except without the addition of an electrochemical
mediator. Such mediators may be advantageous in measurement of the
target analyte, but can interfere with the measurement of analyte
independent signals, such as the electrical resistance change
caused by varying hematocrit levels. In other arrangements, it is
possible that the two measurement regimes could be established at
the same detector, where the reaction conditions change over time.
For example, a first measurement of hematocrit could be made at the
detector before diffusion of the electrochemical redox mediator to
the detector, after which point the detector means is optimised,
and used for, target analyte measurement.
[0049] By providing `live` calibration of every sample tested, at
the point of use, potential variations in the test result arising
from sample effects (such as the presence of interferences,
haematocrit level differences, etc.), environmental factors and
manufacturing imprecision can be determined and the test results
adjusted accordingly.
[0050] Therefore errors in the test results can be substantially
reduced, or even effectively eliminated.
[0051] The calibration may be applied to the unprocessed
measurement of the analyte level in the unadulterated sample and an
analyte level reading then generated from the adjusted analyte
level.
[0052] Alternatively, the internal calibration may be applied to an
analyte level reading generated from the measured analyte level in
the unadulterated sample, thereby correcting it.
[0053] Preferably the clinically relevant analyte is glucose.
[0054] Typically the fluid is whole or fractionated blood.
[0055] The method of the present invention may allow the weighting
given to the `batch calibration` of sensor strips in the
calculation of analyte levels (e.g. blood glucose concentrations)
to be reduced as the `live calibration` weighting is increased. The
batch calibration procedure is generally a lengthy and costly
process. Therefore use of live calibration according to the present
invention could allow the amount of batch testing carried out to be
reduced thus saving manufacturing cost.
[0056] The analyte level reading may be generated by applying a
calibration curve to the analyte level. For example, in the methods
of the present invention, analyte levels (such as blood glucose
concentrations) may be derived from an electrical signal generated
by the sensing chemistry by reference to a calibration curve which
relates analyte concentration to signal magnitude. Such `batch
calibration` curves can be generated based on average performance
for a batch of strips and do not therefore take into account any
particular error related to the individual measurement being made.
The method of the present aspect may allow adjustment of the
calculated analyte concentration for the unadulterated sample by
taking into account the calculated analyte concentration or the
kinetics of the change in analyte level in the calibration sample.
By such a calibration method, errors relating to the individual
measurement being made can be minimised.
[0057] In particular embodiments of the methods of the present
invention, analyte level readings may be generated by applying a
calibration curve to an unprocessed measured analyte level or to an
adjusted analyte level.
[0058] The calibration curve applied to the unadulterated reading
could be different to that applied to the adulterated reading,
reflecting for example a difference in signal response over time.
Additionally, several measurements of the adulterated sample
reading could be taken to establish the kinetics of the dissolution
and diffusion of added analyte to the detector. Information
relating to the sample matrix (for example its temperature or
rheological characteristics) could be inferred from the kinetic
profile and this information used to adjust the unadulterated
reading by making reference to a known relationship between the
sample matrix properties and analytical accuracy effects. This may
be achieved by measuring a single parameter of the kinetic profile
to provide adjustment for all error sources; or where multiple
curve parameters are integrated to provide adjustment for all error
sources; or where different curve features provide different
adjustments for different error sources.
[0059] Preferably the method further includes the step of measuring
the analyte level in one or more further calibration samples of
said fluid at further predetermined times which are later than said
second predetermined time, and the step of adjusting uses all
measured analyte levels to calibrate the reading from the
unadulterated sample. The further measurements may be carried out
in a regular time series (i.e. with a standard interval between
measurements).
[0060] By taking readings of the calibration samples at more than
one point in time, it may be possible to further enhance the
accuracy of the calibrated reading.
[0061] In addition to providing an improvement in accuracy by
adjusting an erroneous measurement, the added analyte response
could be used to determine that a measurement is so inaccurate that
adjustment should not be made and an error reading is generated,
rather than an adjusted reading.
[0062] A further potential application of this aspect is to
determine the condition of a diagnostic test consumable that may be
subject to manufacturing variability or changes due to ageing. In
this application, a sample containing a known amount (which may be
zero) of the analyte is introduced into the detection chamber.
[0063] This may be achieved by effectively "testing" a standardised
sample containing a known amount of the target analyte in a well
characterised sample matrix (`a standard` or `control solution`)
using the method of this aspect (optionally in the device of the
third or fourth aspects below) and obtaining two signals: one from
the unadulterated standard and the other from the adulterated
standard sample containing added analyte. This test generates at
least two signal readings from a series of predicted analyte
concentrations and allows a relationship to be established between
sample analyte concentration and signal. Such a relationship would
reveal the sensitivity of the diagnostic system to sample analyte
as well as the predicted response of the system at zero analyte
concentration. This latter reading can be used as a measure of
diagnostic test quality since it represents the background reading
that is related to non analyte specific response. In the case of
electrochemical enzyme electrodes using redox mediators, a high
background reading can arise from mediator instability. In
practical terms this could be a useful way to test the status of a
set of diagnostic test consumables.
[0064] Where multiple readings from a calibration sample are taken,
the step of adjusting may include calculating one or more rates of
change of the signal measured, or of the calculated concentration
of the analyte. In particular, the step of adjusting may be
performed by inferring or measuring a rate of change in the
concentration of said analyte in the fluid in the detection
chamber.
[0065] The method may further include the step of releasing said
analyte for mixing with said fluid in said chamber. Examples of
ways in which the release of the analyte may be controlled are
discussed in more detail in relation to the third or fourth aspects
below. The step of releasing may include one or more of the
following: applying an electric field to the analyte in the
detection chamber; applying radiation (such as infrared radiation)
to the analyte in the detection chamber; applying heat to the
analyte in the detection chamber; controlled dissolution of a
barrier layer; controlled release from a matrix; controlled release
from a surface.
[0066] The method may further include the step of storing
non-analyte specific information from the measurements
performed.
[0067] In particular, the method may further including the steps
of: storing a record, such as an average or historical data, of
said non-analyte specific information; updating said record after
subsequent measurements; and determining a deviation from said
record in a subsequent measurement.
[0068] The method may further include the step of, if said
deviation is greater than a predetermined amount, alerting the
user.
[0069] Accordingly, it may be possible to identify clearly
erroneous measurements from the non-analyte specific information
and therefore alert the user to the fact that even the adjusted
analyte level is not a correct reading. This may arise as a result
of extremes of temperature exposure during storage before use of
the device, or contamination of the test sample. In other
situations, such determination of deviation may identify when
different users are using the device and therefore different
calibration relationships should be used or applied.
[0070] The method may further include the steps of: recording said
measured analyte levels for a predetermined length of time;
analysing the shape of the measured analyte levels over said
predetermined length of time; selecting, based on said analysis, a
correction algorithm from a plurality of such algorithms, to be
used in said step of adjusting.
[0071] In particular, the step of analysing the shape of the
measured analyte levels may analyse one or more of the following
features of the measured analyte levels: peak height, time to peak
height, absolute value of the measured parameter, and maximum
gradient of transients for different error sources.
[0072] It has been found that different sources of error can affect
the time-dependent variation of measured analyte levels in
different ways. By analysing the shape of the measured analyte
levels over a predetermined length of time it may therefore be
possible to determine the most likely or dominant source(s) of
error and apply appropriate correction in the adjustment step.
[0073] The methods of the present invention preferably also include
the step of introducing said fluid to a flow path, wherein said
fluid flows along the flow path whilst said steps of measuring and
mixing are carried out.
[0074] A possible algorithm that may be used to calculate an
adjusted analyte concentration is:
Gl.sub.adj=(Gl.sub.un.times.Q)/(Gl.sub.cal-Gl.sub.un) [0075] where
Gl.sub.adj, Gl.sub.un and Gl.sub.cal are respectively the adjusted
analyte concentration, the analyte concentration measured in the
unadulterated sample of the fluid, and the analyte concentration
measured in the calibration sample and Q is the known increase in
concentration of analyte in the calibration sample resulting from
the addition of a known amount of the analyte to a known volume of
sample.
[0076] The step of adjusting may include inferring a property of
the sample matrix from the observed signal caused by the added
analyte, or from an analyte independent measurement made under a
different measurement regime, and using this inference to adjust
the unadulterated sample reading. Such inference may particularly
be derived where a series of measurements have been made, which may
allow a property of the sample matrix to be inferred on the basis
of the change in the measured levels over that series.
[0077] The levels measured may be concentrations of the analyte,
and the sample of fluid which is mixed with the predetermined
amount of analyte may be of a known volume.
[0078] In the methods of the present invention, the step of
adjusting may also include making corrections based on one or more
external factors such as ambient temperature or altitude. In this
respect, `external` is used to refer to a correction that arises
from use of an analyte independent measurement method or system
separate from the analyte dependent measurement method or system
(for example a temperature reading provided by a thermistor within
an associated meter). By incorporating information from such
sources with the information derived from the calibration analyte
or different measurement regime reading the effect of external
conditions (such as environmental conditions) on the measurements
can be compensated for.
[0079] The method may be carried out using a device according to
the third or fourth aspects of the invention below, including some,
any or all of the preferred or optional features of that aspect.
However, it will be appreciated that other devices may also be used
in methods according to the present invention.
[0080] A second aspect of the present invention provides a method
for testing, in a portable device, a level of a clinically relevant
analyte in a fluid, the method including the steps of: guiding a
sample of the fluid to a detection chamber containing a
predetermined amount of an analyte; measuring an analyte level in
an unadulterated sample of the fluid; measuring an analyte level in
a calibration sample of the fluid which has mixed (either
completely or in part) with said predetermined amount of said
analyte; and adjusting the analyte level measured in said
unadulterated sample using the analyte level measured in said
calibration sample.
[0081] The method of this aspect may include any individual
optional feature discussed above in relation to the first aspect,
or any combination of some or all such features, without limitation
to essential features of the first aspect which are not part of
this second aspect.
[0082] For example, the method of this second aspect may include
the step of storing non-analyte specific information from the
measurements performed or may include the steps of: recording said
measured analyte levels for a predetermined length of time;
analysing the shape of the measured analyte levels over said
predetermined length of time; and selecting, based on said
analysis, a correction algorithm from a plurality of such
algorithms, to be used in said step of adjusting.
[0083] At its broadest a third aspect of the present invention
provides a device for measuring levels of clinically relevant
analytes, such as glucose, in a fluid and calibrating that
measurement as a result of subsequent measurement of a calibration
sample by the same detector.
[0084] A third aspect of the present invention provides a device
for measuring a level of a clinically relevant analyte in a fluid,
including: a flow path for conducting said fluid through the
device; a detection chamber arranged on said flow path; and
detector means arranged to detect analyte levels in the fluid in
said chamber, wherein: said detection chamber contains a
predetermined amount of an analyte such that that analyte mixes
with fluid in the detection chamber to form, at the detector means,
a calibration sample of the fluid at a time (or series of times)
after the arrival of the fluid in said detection chamber, and said
detector means is arranged to detect a first analyte level of an
unadulterated sample of the fluid at a first time which is before
the formation of said calibration sample and to detect a second
analyte level of said calibration sample at a second time which is
after the formation of said calibration sample.
[0085] Any of the following optional features (including
combinations of some or all such features) may be included in the
device of this third aspect.
[0086] Preferably the analyte in said predetermined amount is the
same as said clinically relevant analyte. As a result the
measurement of both the unadulterated sample and the calibration
sample can be carried out by the same method, and so errors in that
method can be detected and adjusted for. Alternatively a different
calibration analyte may be used as described in relation to the
method of the first aspect above.
[0087] As both the unadulterated sample and the calibration sample
are measured in the detection chamber, they can both be measured by
a single detector. Thus there is a temporal separation between the
measurement of the unadulterated sample and the measurement of the
calibration sample and any errors due to variation in the detectors
used for measuring the levels in the calibration and unadulterated
samples can be avoided.
[0088] Furthermore, as both the unadulterated sample and the
calibration sample are formed from the same volume of fluid, there
is no requirement for an additional amount of fluid to be provided
for the analysis.
[0089] To simplify the device and keep manufacturing costs low, it
is preferable that the device has a single detector means, a single
flow path or both.
[0090] The use of a single detector and flow path has a number of
advantages including the ability to achieve the enhanced accuracy
of live calibration without any requirement to increase the
required sample volume and size of the device and compatibility of
the consumable element with pre-existing meters or reader devices.
Moreover, the use of a single detector and sample chamber can
eliminate any errors that might arise from differences between
detectors and/or dimensions of sample chambers.
[0091] Typically the predetermined amount of analyte is provided in
solid form and the mixing to form the calibration sample occurs
when the analyte dissolves in the fluid.
[0092] Preferably the clinically relevant analyte is glucose.
[0093] Typically the fluid is blood.
[0094] The predetermined amount of analyte in the detection chamber
may be arranged in a number of ways which may improve the accuracy
of the calibration.
[0095] In one arrangement the predetermined amount of analyte is
located at a position within the detection chamber remote from the
position of the detector means.
[0096] For example, the predetermined amount of analyte may be
located on an opposing surface of the interior of said chamber to
the position of the detector means.
[0097] In one configuration, the detector includes a working
electrode and said predetermined amount of analyte and the working
electrode of the detector are located on opposing surfaces of the
detection chamber.
[0098] In one arrangement of this configuration, at least part of
said predetermined amount of analyte is located directly opposite
at least part of said working electrode. In other words, when
viewed along an axis perpendicular to the surfaces, the area
containing the predetermined amount of analyte and the footprint of
the working electrode overlap.
[0099] In another arrangement of this configuration, there are two
detector means and the distance between said opposing surfaces is
different in the region of each of said detector means. This
arrangement can allow the rate or time of arrival of analyte at the
respective detector means to be different as a result of the
different separations.
[0100] In this arrangement the diffusion of the predetermined
amount of analyte from its position, through the fluid, to the
detector means provides sufficient time separation for the
detection of the analyte level in the unadulterated sample before
subsequent detection of the analyte level in the calibration
sample. Typically this time separation is at least 0.1 seconds,
preferably at least 0.5 seconds, more preferably at least 1 second.
Such time separations have been achieved with distances of at least
100 .mu.m between the detector means and the analyte, but may be
achieved with distances of at least 40 .mu.m.
[0101] In an alternative or additional arrangement, the chamber has
an inlet port connected to said flow path and said predetermined
amount of analyte is located at a position in said chamber which is
further from said inlet port than the position of said detector
means in said chamber. In this arrangement, the additional time
taken for the fluid to reach the predetermined amount of analyte
after it has already reached the detector means allows a time
separation between the measurements.
[0102] Additionally, there could be a series of added analyte
locations or formulations and the difference between observed and
expected added analyte signal could be used to infer information
about the test sample matrix that could be used to make a
correction to the analyte measurement.
[0103] In a further alternative or additional arrangement, the
predetermined amount of analyte is contained in a controlled
release formulation. Many forms of controlled release formulation
are known and can be used for securing the analyte, including (but
not limited to) controlled release matrices; multilayer
formulations; encapsulation; crystalline structures; or a film
coated formulation.
[0104] Use of a controlled release formulation may have the same
effect as locating the predetermined amount of analyte in a
particular position within the detection chamber as it allows
control of the time taken for the effects of that predetermined
amount of analyte to reach the detector means. The use of a
controlled release formulation may, of course, be done in
conjunction with specific locations of the analyte.
[0105] Controlled release formulations can work in a variety of
ways. In some controlled release formulations, the formulation will
not release its contents until a certain time after contact with
the fluid. This can provide the desired time separation between
detection of the analyte levels in the unadulterated and
calibration samples.
[0106] In other controlled release formulations, the formulation
does not release anything until it has been activated. Such
activation generally acts to change the structure of the
formulation thereby allowing release. This may be by application of
an energy source to either change the temperature of the
formulation or to promote mixing.
[0107] In one specific arrangement, the controlled release
formulation is configured so as to contain the analyte in a number
of micelles. The application of an electrical field to the
formulation causes the micelles to be disrupted, thereby releasing
their payload of analyte. To achieve this the detection chamber may
contain one or more electrical elements.
[0108] Other activation methods for controlled release formulations
include irradiation (for example with infrared light) and
heating.
[0109] In one arrangement, said predetermined amount of analyte is
located on a substrate at a predetermined position in relation to
the position of the detector means.
[0110] The flow path and detection chamber may be formed by a
plurality of structural components and said substrate may form part
of at least one of said structural components.
[0111] This arrangement allows the predetermined amount of analyte
to be located on the substrate which is then incorporated into the
device.
[0112] In particular arrangements, the substrate may be arranged or
manufactured so as to control the location of the analyte on the
substrate. For example, the predetermined location of the analyte
may be within an area on said substrate which has a different
surface energy to other areas on the same substrate.
[0113] There are a number of ways of providing an area on the
substrate with a different surface energy. For example, the
predetermined location of the analyte may be in a hydrophilic well
which is surrounded by a hydrophobic area. In many cases the
analyte will be deposited onto the substrate in solution, in which
case the provision of a hydrophilic well within an otherwise
hydrophobic area of the substrate will enable precise positioning
of the analyte solution, avoiding spreading of the solution across
the hydrophilic area.
[0114] Alternatively the substrate on which the predetermined
amount of analyte is located may be generally hydrophobic and the
formulation containing the predetermined amount of analyte may
provide a hydrophilic surface that facilitates capillary fill of
the flow path and detection chamber.
[0115] In another arrangement the substrate on which the
predetermined amount of analyte is located is generally hydrophobic
and is covered by a layer of hydrophilic material which forms a
wall of the flow path. This hydrophilic covering material may be
arranged to dissolve on contact with the test sample, exposing the
added analyte and allowing it to mix with test sample.
[0116] In certain embodiments, the predetermined amount of analyte
is contained within a formulation containing a thickening agent
such as PVP, PVA, alginate, CMC, HPMC, pullulan etc.
[0117] In one embodiment, the predetermined amount of analyte is
positioned in a line across the width of the flow path or detection
chamber in a direction orthogonal to the direction of flow through
the flow path or into the detection chamber.
[0118] This arrangement of the analyte allows the device to take
account of manufacturing tolerances in the width of the flow path
or detection chamber. As an increase or decrease in the width of
the flow path of detection chamber will result in an increase or
decrease in the volume of fluid in the flow path or detection
chamber, positioning the analyte in this manner results in a
corresponding, preferably proportional, increase or decrease in the
amount of analyte in said predetermined amount.
[0119] In other arrangements the flow path and detection chamber
are formed by a plurality of structural components and said
predetermined amount of analyte is contained within the device on a
substrate which is separate from said structural components.
[0120] Variation of the formulation of added analyte and factors
such as the spatial geometry of the sample chamber, flow patterns,
surface energy modification can be applied together to tune the
temporal separation of the measurement of unadulterated and
adulterated sample such that discrimination between the
measurements is optimised in the shortest overall measurement time.
In this manner the time separations set out above may be
achieved.
[0121] In one arrangement, the detection chamber contains 2 or more
discrete predetermined amounts of the analyte. Each of these
discrete amounts of analyte may be arranged to dissolve into fluid
in the detection chamber at a different rate, thereby affecting the
concentration profile of the fluid against time after it enters the
chamber in a known way. Alternatively or additionally each discrete
amount of analyte may be arranged to dissolve into fluid in the
detection chamber at a different time, thereby affecting the
concentration profile of the fluid against time after it enters the
chamber in a known way. In this arrangement, each discrete amount
of analyte could be a different chemical species, for example one
could be the target analyte and another, the calibration
analyte.
[0122] The device may further comprise means for applying a
variable electrical field to the flow path or to the detection
chamber. The variations in electrical field could be used to
influence the rates of mixing of components within the calibration
sample.
[0123] The predetermined amount of analyte may be formulated so
that it undergoes an exothermic reaction when dissolving into the
fluid to form the calibration sample. This arrangement could
quicken rates of mixing within the calibration sample.
[0124] The flow path of the device is preferably arranged to reduce
propensity for pendant drop formation when said fluid is being
added to the device. This can allow the amount of fluid added to
the device to be precisely controlled.
[0125] In certain embodiments, the device is arranged to accept a
precise volume in the detection chamber and has an overflow chamber
to accommodate amounts of fluid greater than the capacity of the
detection chamber. This allows the quantity of fluid in the
detection chamber to be controlled irrespective of the quantity of
fluid that is introduced to the device.
[0126] In certain embodiments, said predetermined amount of analyte
is formed on a surface of the detection chamber and covered by a
further layer, the further layer preventing interaction between the
analyte and the fluid and wherein a predetermined area of the
analyte is exposed to the interior of the detection chamber by a
hole in said further layer. This arrangement has the advantage that
the amount of dosed analyte exposed to sample fluid with which it
mixes is related to the hole dimensions rather than the total
amount of analyte deposited. This may provide for a more precise
amount of analyte to be mixed with the sample fluid. Additionally
in this arrangement, the analyte is not directly exposed to the
flow of the fluid through the flow path and therefore is less
likely to be translocated along the flow path as the fluid moves
along it. The further layer may be formed from materials which are
optimised for causing capillary fill of the flow path.
[0127] In certain arrangements, the device includes at least 2
detector means, wherein a first detector means is optimised for
measuring the clinically relevant analyte, and a second detector
means is optimised for measuring a non-analyte-dependent signal,
such that the non-analyte-dependent signal can be used to obtain
information about the analyte level measured by the first of said
detector means.
[0128] In particular, the second detector means may be arranged to
detect a quantity related to the diffusivity of the clinically
relevant analyte.
[0129] In one arrangement, the first detector means includes a
redox mediator and the second detector means does not include said
redox mediator, and the detector means are arranged such that the
measurement of the non-analyte-dependent signal is made before said
redox mediator is able to diffuse to the second detector means.
[0130] The detector means of the device may be arranged to detect a
third (and optionally subsequent) analyte level of additional
calibration samples of the fluid at further predetermined times.
This can allow a profile of the change in analyte level over time
to be recorded and may assist in improving the accuracy of the
adjusted reading of the analyte level further.
[0131] The detector means of this aspect may include at least one
enzyme electrode or other detection means such as optical
(absorbance or light scattering based) or spectral (fixed or
multiple wavelength based).
[0132] The device of this aspect may further include a transducer
or processor for processing signals from said detector means to
produce an analyte level reading. The analyte level reading may be
determined before or after a live addition calibration process is
applied, as described in relation to the first aspect above.
[0133] Preferably the transducer or processor is adapted to produce
an analyte level reading by adjusting the analyte level detected in
the unadulterated sample according to analyte level(s) detected in
said calibration sample.
[0134] Preferably the flow path operates to draw the fluid through
the device by capillary action. This may allow the amount of fluid
flowing through the device to be known, enabling determination of
the increased concentration of analyte in the calibration sample
arising from mixing with a known amount of the analyte or may allow
the fluid path to be well defined, so that diffusion times of added
analyte can be well characterised. This arrangement may also reduce
the possibility of user error contributing to an error in the
detected analyte levels.
[0135] A fourth aspect of the present invention provides a device
for measuring a level of a clinically relevant analyte in a fluid,
including: a flow path for conducting said fluid through the
device; a detection chamber arranged on said flow path; and
detector means arranged to detect analyte levels in the fluid in
said chamber, wherein: said detection chamber contains a
predetermined amount of an analyte such that that analyte mixes
with fluid in the detection chamber to form, at the detector means,
a calibration sample of the fluid, and said detector means is
arranged to detect a first analyte level of an unadulterated sample
of the fluid and to detect a second analyte level of said
calibration sample.
[0136] The device of this aspect may include any individual
optional feature discussed above in relation to the third aspect,
or any combination of some or all such features, without limitation
to essential features of the first aspect which are not part of
this fourth aspect.
[0137] For example, the device of this second aspect may include at
least 2 detector means, wherein a first detector means is optimised
for measuring the clinically relevant analyte, and a second
detector means is optimised for measuring a non-analyte-dependent
signal, such that the non-analyte-dependent signal can be used to
obtain information about the analyte level measured by the first of
said detector means. Alternatively or additionally the chamber of
the device of the second aspect may have an inlet port connected to
said flow path and said predetermined amount of analyte is located
at a position in said chamber which is further from said inlet port
than the position of said detector means in said chamber.
[0138] The devices of the present invention may further include one
or more sensors, which may be connected to the transducer or
processor. The device, for example in the transducer or processor
may process signals from the sensor(s) when producing an analyte
level reading. The sensors may be, for example, temperature
sensors.
[0139] The devices of the present invention are preferably
portable, and so can be carried by the user and readily available
for use in a wide variety of situations.
[0140] Embodiments of the present invention will now be described
with reference to the accompanying drawings, in which:
[0141] FIGS. 1A and 1B are schematic illustrations of part of a
blood glucose monitoring device according to a first embodiment of
the present invention before the flow of blood into the detection
chamber and after the additional glucose has been dissolved and
diffused;
[0142] FIGS. 2A and 2B are schematic illustrations of part of a
blood glucose monitoring device according to a second embodiment of
the present invention before the flow of blood into the detection
chamber and after the additional glucose has been dissolved and
diffused; and
[0143] FIGS. 3A and 3B are schematic illustrations of part of a
blood glucose monitoring device according to a third embodiment of
the present invention before the flow of blood into the detection
chamber and after the additional glucose has been dissolved and
diffused to the nearest detector.
[0144] A first embodiment of the present invention is shown
schematically in FIGS. 1A and 1B. FIGS. 1A and 1B show a detection
chamber 2 which is connected to a flow path (not shown) which is
arranged to guide the fluid being analysed to the detection chamber
2 via an entry port 10. FIG. 1A shows the chamber 2 before entry of
the fluid (as indicated by the arrow F). FIG. 1B shows the chamber
2 after the fluid 8 has filled the chamber 2 and the analyte 4 has
dissolved in the fluid 8.
[0145] In the embodiment of FIG. 1A the predetermined amount of
analyte 4 is provided in the detection chamber 2, and is spatially
removed from a detector 6.
[0146] The detector 6 is an enzyme electrode such as a glucose
oxidase or glucose dehydrogenase based electrode. A standard form
of an enzyme electrode is made up of a number of layers. The first
is a conductive track, on top of which is the layer containing the
enzyme (e.g. glucose oxidase) and, potentially, redox mediators
(e.g ferricyanide or ferrocene). Above this there may be a mesh
which serves to spread out the blood or other fluid being tested. A
membrane can also be provided above the electrode surface to
physically, chemically or electrically prevent interferents from
reaching the enzyme electrode. The detector may include a counter
and/or a reference electrode. However, where there is more than one
detector, the counter and/or reference electrode may be common
between the detectors.
[0147] In this embodiment, the time taken for diffusion of the
analyte 4 through the fluid 8 being analysed to the detector 6
provides a time window in which the measurement of the
unadulterated sample can be made. The second measurement of the
calibration sample can be made after a time by which it is known
that diffusion of analyte 4 to the detector 6 will have
occurred.
[0148] As shown in FIG. 1A, the added analyte 4 is situated on the
under surface of the lid of the chamber 2 and the detector 6 is
situated on the bottom surface of the flow path. The time window
between when the detector 6 is measuring an unadulterated sample of
the fluid 8 and a calibration sample of the fluid 8 can be adjusted
by arranging the position of the added analyte 4 in relation to the
entry port 10 and therefore the arrival of the fluid into the
chamber. For instance, situating the added analyte 4 "downstream"
of the detector 6 (i.e. further from the entry port 10 than the
position of the detector 6) as shown in FIG. 1A) allows a first
reading to be initiated before the added analyte 4 has been
contacted by the fluid entering the chamber 2.
[0149] In developments of this embodiment, the detection chamber 2
may contain a number of elements that, together or in isolation,
adjust the time window between unadulterated measurement and
measurement of calibration sample with added analyte 4 mixed in.
For example, the detection chamber 2 may contain one or more
microstructural elements (not shown) that promote turbulent flow of
the sample fluid in the region of the undissolved additional
analyte 4. The presence of these elements would have the effect of
enhancing mixing and reducing the time for the added analyte to
reach the detector 6. Alternately, turbulence or specific flow
patterns could be achieved through modification of the surface
energy of different parts of the chamber 2.
[0150] It has been found that, for the measurement of glucose in
blood, in an arrangement such as that set out in the above first
embodiment, it is possible to take a reading of the level of
glucose in the blood approximately 0.5 seconds after the arrival of
the fluid in the detection chamber which is unaffected by the
presence of the additional glucose. A subsequent reading of the
level of glucose in the blood can be taken after 5 seconds, by
which time some or all of the additional glucose 4 has dissolved in
the blood and has also diffused to the detector 6.
[0151] FIGS. 2A and 2B show a second embodiment of the present
invention. Where components are similar to those shown in the first
embodiment above, similar reference numerals are used (e.g. the
detection chamber in the two embodiments is denoted by 2 and 2'
respectively).
[0152] In this embodiment the detection chamber 2' has two
detectors 6' and 7'. Both detectors are positioned on the opposite
surface of the flow path to the added analyte 4'. Detector 7' is
positioned closer to the added analyte 4' than detector 6' so that
added analyte diffusing away from its original position after
arrival of fluid in the chamber 2' reaches detector 7' first.
[0153] By selecting the relative dimensions of the chamber 2' and
the positioning of the detectors 6', 7', it is possible to create
an arrangement where the detector 7' measures the effect of the
added analyte at a time when added analyte has not substantially
diffused to detector 6' so that the latter detector measures only
the analyte intrinsically present in the unadulterated sample. By
this arrangement, both adulterated and unadulaterated sample
measurements can be made within the same chamber 2' at the same
time, by making use of the separation in distance between the two
or more detectors. This configuration is shown schematically in
FIG. 2B where the hatched area represents the intrinsic analyte
concentration in the fluid 8' and the dotted/stippled areas
represent concentrations resulting from dissolution and/or
diffusion of the added analyte 4'
[0154] It has been found that if detector 7' is positioned opposite
added analyte at a distance of less than 200.mu., and detector 6'
is also arranged on the opposing surface of the flow path but
removed in distance along the direction of flow by at least
300.mu., separate measurement of adulterated and unadulterated
samples is possible within a time window of 1.about.20 seconds
after test sample has filled the capillary. Because the added
analyte has not diffused to form a homogeneous concentration of
analyte within the entire test sample volume, the measurement of
analyte level at detector 7'can be a measure of a transient level
(e.g. concentration) or the rate of change of a level.
[0155] In an alternative arrangement of this second embodiment the
added calibration analyte may be a different species to the target
analyte. Both the calibration analyte and the target analyte can be
measured at the detector 7'. For example, the calibration analyte
could be directly oxidised at the detector producing a signal that
would indicate diffusivity of that species in the test sample and
this in turn could be used to infer information about the test
sample matrix and to adjust the reading of the target analyte
level.
[0156] Alternatively, the added species could be a substance that
inhibits the analyte specific detection reaction. For example, an
inhibitor of glucose dehydrogenase or glucose oxidase could be
added above the calibration detector in a blood glucose monitoring
strip. Inhibition of the glucose specific response at this detector
as the substance diffuses to it would allow measurement of the non
analyte specific background current that may occur in the presence
of electrochemical interferents.
[0157] In a further arrangement, based on the second embodiment,
one of the detectors 6', 7' could have a different detection
chemistry, for example by changing the construction of the
detector. For example, the enzyme electrode 6' is arranged to
detect the target analyte, but the second electrode 7' may lack
components which are present in an enzyme electrode, but which
would interfere with a non analyte dependent measurement. For
example, a redox mediator may be present as part of the detector 6'
but be absent in detector 7'. Because of the diffusion time
required to transport any mediator from detector 6' to detector 7',
there will be a time window where an electrochemical measurement of
a sample property, such as hematocrit level, can be made in the
absence of the mediator.
[0158] As a result of the absence of the mediator in detector 7' it
is possible to make the electrochemical measurement of hematocrit
with a high or higher sensitivity than otherwise. This measurement
could be used in combination with information derived from the
added analyte measurement to accurately adjust the reading of the
target analyte level.
[0159] This arrangement can therefore provide for a sample
parameter specific correction (such as haematocrit), based on the
optimised non-analyte dependent measurement, together with a
correction based on the measured added analyte specific
response--which is indicative of the overall uncorrected detector
response to the analyte.
[0160] In a further embodiment of this arrangement, it may not be
necessary to have an added analyte, since the sensitive non-analyte
dependent measurement may provide sufficient opportunity for a
substantial correction of the analyte level. This embodiment could
be beneficial where the measurement error is substantially
attributable to one, or a small defined number of, sample
parameters that could be measured by the optimised non-analyte
dependent measurement.
[0161] In a third embodiment of the present invention, illustrated
schematically in FIGS. 3A and 3B, the time resolution of the
initial and later, adulterated readings is provided by application
of specific formulation techniques to the additional analyte. Where
components are similar to those shown in the first embodiment
above, similar reference numerals are used (e.g. the detection
chamber in the two embodiments is denoted by 2 and 2''
respectively). Again, FIG. 3A shows the arrangement of the detector
chamber 2'' before entry of the fluid to be tested. FIG. 3B shows
the arrangement of the detector chamber 2'' after the chamber has
filled with fluid and the additional analyte 4'' has dissolved in
the fluid 8'.
[0162] In this embodiment, the added analyte 4'' and the detector
6'' are not significantly spatially separated (in the arrangement
shown in FIG. 3A, there is a small separation, but the added
analyte 4'' could even be formed as part of the detector 6'' or
located on or around the detector 6''). Instead of the separation
of the first embodiment above, the kinetic dissolution or release
profile of the added analyte 4'' is arranged or chosen such that a
first reading can be made before dissolved added analyte 4'' is
apparent at the detector 6''.
[0163] This arrangement could be achieved using a range of
formulations for the added analyte 4'', for example by formulating
the analyte 4'' in a controlled release matrix (e.g a colloid), in
a multilayer formulation, in an encapsulation including a micelle
or a liposome based formulation, different crystalline structures,
different coating film thickness etc.
[0164] One specific way of achieving a controlled release of the
analyte 4'' of this embodiment is to initiate a change in the
structure of the formulation by application of an energy source to
either change the temperature or promote mixing. For example, where
the analyte 4'' is stored in micelles, an electric field applied to
the sample at a certain time would cause micelles to be disrupted,
thus releasing their payload of analyte. This arrangement could be
facilitated by the presence of electrical elements (not shown)
within the sample chamber 2''. However, other activation methods,
such as use of infrared illumination could also be used to activate
the formulation.
[0165] In this embodiment a plurality of discrete portions of the
analyte 4'' are provided in the detection chamber 2''. The
plurality of discrete portions of analyte may be provided to
improve the uniformity of the dissolution and diffusion of the
analyte in the fluid.
[0166] Alternatively, each discrete portion of analyte 4'' may be
arranged to release its payload of analyte at a different time (for
example the nature of the formulation for each portion of analyte
may be chosen to achieve a particular release profile, or so that
the release of the analyte is triggered by a different source or a
different duration in the same environment, or triggered by the
release of a component from an earlier release event) thereby
creating a known change in the concentration of analyte in the
fluid 8'' and detected by the detector 6'' which can then be used
in the calibration of the reading from the unadulterated
sample.
[0167] There are a number of ways that signals arising at detectors
from the addition of added analyte to the sample chamber can be
used to adjust the signal from the unadulterated sample to improve
the accuracy of the measurement. For example the unadulterated and
adulterated samples can be considered to be discrete measurements
where the adulterated sample contains a stable known amount of
added analyte and this is used to correct the unadulterated signal.
In this modality, the relationship between analyte level and
measured signal may be different at the two time points, but this
difference can be pre-characterised and used to accurately measure
the intrinsic and total (intrinsic plus added) analyte levels at
respective time points.
[0168] Alternatively, the rate of dissolution or diffusion of the
analyte in the particular test sample matrix could be inferred by
single measurement of a transient analyte level or by a series of
measurements over time. This information could be used to apply a
correction factor to the unadulterated sample, for example by
reference to a `lookup table` in the processor which contains
information about the relationship between dissolution and/or
diffusion rates and test sample measurement accuracy.
[0169] This latter method of adjustment may be used with a further
development of the embodiments in which multiple discrete amounts
of analyte are positioned in the detection chamber. These discrete
amounts could be spatially separated throughout the chamber to
provide a desired release profile or detection profile at the
detector, or could be provided in a range of different formulations
with, for example, differing time release profiles.
[0170] This method has the ability to correct for several error
sources in test systems where the rate of diffusion to the detector
is an important aspect of the correct functioning of the system.
For example, in blood glucose monitoring, hematocrit levels and
test temperatures both affect the diffusivity of glucose in blood
and thereby inaccuracies can arise if there is deviation in
hematocrit level or test temperature from the conditions under
which the system has been calibrated.
[0171] Since this particular method of correction involves a step
where properties of the test sample are inferred from measurement
of diffusivity, it is possible that this information could
additionally be used for other means. For example, a meter could
keep a record of blood diffusivity as an indicator of haematocrit
and/or plasma viscosity and determine if a person's hematocrit
level is changing. This could be an early indicator of a change in
a person's state of health. Alternatively, if the meter detected a
significant (step) change in blood diffusivity, it could prompt the
user to determine whether he or she is the normal user of that
system. This can be a useful intervention because it could warn
potential users of the cross contamination risks associated with
using someone else's meter or the meter could store test data in a
separate file, preventing any potential confusion arising from
interpretation of trending data, or use different baseline
configurations such as look up tables depending on the user.
[0172] In one embodiment of a method of the present invention, the
internal calibration is achieved by incorporating a predetermined
amount of glucose into the test sample and measuring the blood
glucose response in both the presence and in the absence of the
added glucose. The difference between the signal generated by the
two samples is used to modify the unadulterated sample result.
[0173] An example situation is given below, in which the numbers
have been chosen for purely illustrative purposes:
TABLE-US-00001 Normal Haematocrit range (Hct) High Hct Low Hct
Unadulterated 5 4 6 test result (mM) Calibration 10 8 12 result
(mM) Adjusted 5 5 5 result (mM)
[0174] The adjusted concentration result is obtained as:
Gl.sub.adj=(Gl.sub.un.times.Q)/(Gl.sub.cal-Gl.sub.un) [0175] where
Gl.sub.adj, Gl.sub.un and Gl.sub.cal are respectively the adjusted
analyte concentration, the analyte concentration measured in the
unadulterated sample of the fluid, and the analyte concentration
measured in the calibration sample and Q is the known increase in
concentration of analyte in the calibration sample resulting from
the addition of a known amount of the analyte to a known volume of
sample.
[0176] In an alternative embodiment, the measurement of a level of
added analyte can be used to infer a property of the test sample
matrix and this knowledge can be used to correct the analyte sample
level based on a known relationship between that property and error
that it might cause. For example, a blood glucose monitor may
determine a blood glucose reading of 6 mM, when the true, plasma
referenced value is 5 mM, if the hematocrit level of the blood test
sample is unexpectedly low at, say, 30%. Under these conditions,
diffusion of added glucose to a detector may be faster than under
conditions of normal hematocrit (usually 40-50%). Hence the rate of
change of glucose concentration arising from the added glucose
could be 2 mMs.sup.-1 instead of 1 mMs.sup.-1 expected under normal
conditions. A known relationship between observed rate of change of
glucose concentration from added glucose and hematocrit could
reveal that the test sample has a hematocrit value of 30%. A
further known relationship, between hematocrit and error causation
could suggest that 30% hematocrit causes readings to be
artificially raised by 20% and so this would enable the system to
correct the erroneous reading of 6 mM to an accurate reading of 5
mM.
[0177] In this case, the level of calibration analyte measured
could be a transient flux or concentration or a rate of change in
flux or concentration. In this embodiment, it is possible that the
calibration analyte and the target analyte are the same chemical
species, or they may be different species.
[0178] These illustrations demonstrate how a source of error can be
eliminated from blood glucose measurements by using the internal
calibration approach. In the examples given, the unadulterated test
result varies as a consequence of different Hct levels in the test
blood, despite the fact that the true glucose concentration is 5 mM
in all cases. However the effect of the Hct level can be determined
either by observing the difference in measured concentrations
between the unadulterated sample and the calibration sample, to
which a known increase in concentration has been effected or by
measuring a difference between observed and expected levels from
the added analyte measurement. Once the extent of the interference
arising from Hct has been gauged, it is a relatively
straightforward matter to correct for this effect. This
illustration uses Hct as an example source of error, but the same
rationale applies to other interferences or sources of error.
[0179] Other, more complex, algorithms could be used to modify the
test result. For example weightings could be given to the internal
live addition calibration and batch calibration in the calculation
of glucose concentrations, for example:
Calculated glucose concentration=(glucose concentration derived
from batch calibration+adjusted glucose concentration derived from
live calibration)/2.
[0180] It is also possible that the shape of the curve of measured
signal over time (`the transient`) will vary depending on the
source of the error. For example, changes in temperature can affect
both the rate of diffusion of analyte to the detector and the rate
of reaction and signal generation at the surface, whilst hematocrit
changes would primarily affect analyte diffusivity. In order to
make an effective correction, it may therefore be desirable to
understand the nature of the error source as well as the magnitude
of the error and to use this knowledge to select an appropriate
correction algorithm. This could be achieved by characterising
features such as peak height, time to peak height, maximum gradient
etc of transients for different error sources. Such features could
be apparent in the unadulterated or adulterated sample
measurements, or a combination thereof.
[0181] Typically the detection chambers 2, 2', 2'' as shown in
FIGS. 1, 2 and 3 above form part of a sensor strip which has a
capillary fill mechanism and a capillary flow path from a receiving
area where the fluid to be tested is placed to the detection
chamber 2, 2', 2''.
[0182] Connectors (not shown) from the detectors 6, 6', 6'' are
provided in all embodiments for connection of the device to a
transducer (not shown) which converts the signals from the
electrodes into readable results and applies the appropriate
algorithm(s) to calibrate the results.
[0183] In all of the above embodiments, it may be desirable that
the amount of analyte 4, 4', 4'' used for the calibration sample is
accurately and precisely known. In order to achieve this, any one
of a number of techniques may be used to apply the analyte 4, 4',
4'' to the device. The analyte could be applied either in a
solution form (where it could be dissolved in either an organic or
non-organic solvent) or as a solid, neat or in combination with
other formulation ingredients. Examples of techniques for applying
the analyte are: coating technologies (e.g. slot die, wire wound
rod or spraying), printing technologies (e.g. slot-die coating,
ink-jet, screen, gravure or flexographic printing), dispensing
technologies (e.g. positive displacement, aspirate and dispense or
piezoelectrically driven), metering of glucose-loaded beads,
deposition of glucose onto a solid-phase substrate such as paper or
string.
[0184] In some of the above embodiments it may also be important to
accurately determine or know the location of the added analyte.
This can be achieved by well know deposition or printing
techniques, such as the use of drop on demand dispensing or gravure
printing. Additionally, the location of the added analyte could be
controlled by modifying the surface energy of the substrate onto
which analyte is added. For example, a hydrophobic ring could be
formed around the target area for the analyte dosing so the dosed
analyte does not extend beyond that ring and dries in a
predetermined location.
[0185] For the first embodiment above, it is also preferable that
the calibration analyte 4 mixes quickly and repeatably with the
fluid sample once the fluid comes into contact with the analyte.
This can be achieved through the placement pattern of the analyte 4
in the detection chamber 6.
[0186] Adjusting the formulation of the added analyte solution
deposited into the flow path can also increase the rate of
dissolution. Formulating with fast dissolving substances that
increase the viscosity of the coating or deposition solution is
particularly advantageous, since it enables good control of coating
or deposition. Polymers including pullulan, alginate, polyvinyl
pyrrolidone, polyvinyl alcohol, polyethylene glycol, caboxymethyl
cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose,
polyacrylates and natural gums can be used for this purpose,
possibly also in combination with a plasticizer such as
polyethylene glycol and sorbitol and a surfactant such as Triton
X-100. Particular techniques which may be applied are micronisation
of the glucose powder, co-formulation into rapidly re-suspending
hydrocolloids forming a membrane, co-formulation with ingredients
generating effervescence when re-suspended or freeze-drying to form
a high porosity stable crystalline structure.
[0187] In some of the above embodiments, it may be desirable to
separate different zones within the flow path to prevent
inadvertent mixing before sample addition. For example, where there
is a detector optimised for a non-analyte dependent measurement
(e.g. hematocrit) in addition to a detector optimised for analyte
detection, the areas of the flow path could be effectively kept
separate by covering different detection with a film forming agent
of low friability, such as a pullulan film, possibly with added
plasticizers and surfactants.
[0188] For all the above embodiments, it is preferable that the
rate of arrival of added analyte at the detector is insensitive to
normal deviations in the manufacturing processes used for making
the consumable test strip. One way to minimise sensitivity to
position or absolute amount of the added analyte is to dose the
analyte onto the substrate used to form the lid or base of the flow
path as a continuous line or series of small discrete dots forming
a line orthogonal to the flow direction. Thus when the substrate
containing the dosed analyte is formed into a flow path surface and
strips are cut out, the amount of dosed analyte exposed to the test
sample is proportional to the flow path width. In this way,
tolerance issues that could arise when a single discrete dot of
analyte is dispensed onto the substrate with the possibility that a
proportion of this dot is outside the part of the substrate that
forms part of the flow path are avoided. Equally, the analyte could
be uniformly coated onto the surface of the substrate.
[0189] In addition to dosing the analyte in a line orthogonal to
the flow direction or as a uniform coating, the shape of the
detector used to measure added analyte levels can be optimised to
reduce sensitivity to manufacturing deviations. For example, where
the analyte is dosed in a line orthogonal to the intended flow, it
will be advantageous to shape the working electrode part of the
detector in the flow path such that it has a longer dimension in
the same direction as the flow than it does across the flow path
and to position it such that the longer dimension extends beyond
the edges of where the analyte line is positioned. In this way, the
measurement of the dosed analyte becomes tolerant to variations in
both the position of the dosed analyte and the position of the
detector.
[0190] The precise details of the construction and manufacture of
sensor strips is well known in the art, and the present invention
can be embodied in sensor strips which are made from a wide variety
of materials and by a wide variety of methods.
[0191] One example of a sensor strip is described here, purely for
illustrative purposes. The strip has overall dimensions of
30.times.10.times.2 mm. The substrate and cover defining the
capillary flow path are comprised of a flexible plastic, and the
capillary cover is adhered to the base substrate by means of a
pressure sensitive adhesive. Electrode tracks are formed from
conductive screen printing inks or from sputtered then etched
metals and active electrodes are manufactured by screen printing or
dispensing an enzyme solution (potentially including a redox
mediator) onto an underlying conductive track. A counter electrode
forms a circuit and this may have mediator and enzyme at least
partially overlaid. Some systems have a separate reference
electrode. Generally such products are single use disposable
products.
[0192] The readings from the detector may be used to provide an
analyte level reading. The live calibration using the measurement
from the calibration sample or analyte may be applied directly to
the unprocessed measurements from the detector measuring the
analyte level (in the unadulterated sample in the first, second and
third embodiments), or applied to an analyte level reading which
has been derived from the unprocessed measurements.
[0193] For example, the sensor strip of an embodiment of the
present invention may be connected to a transducer which produces
analyte level readings from an electrical input from an enzyme
electrode detector by applying a calibration curve. The reading
obtained may then be adjusted and corrected depending on the
measurement from the calibration sample.
[0194] Alternatively, the electrical signal from the enzyme
electrode detector may be adjusted or corrected as a result of the
measurement from the calibration sample or analyte to create a
corrected electrical signal which is then used by a transducer or
processor to produce a corrected analyte level reading, for example
by applying a calibration curve.
[0195] Although some aspects of the above description have been
made with specific references to the testing of glucose in blood,
it will be clear to the person skilled in the art that the
principles could be applied to other clinically relevant analytes
such as indicators of cardiovascular, liver and kidney function
(e.g. cholesterol, haemoglobin, electrolytes, metabolites),
infectious agents (e.g. viruses, bacteria), disease state
indicators (e.g. C-reactive protein, antibodies, cellular
signalling factors, hormones) and therapeutic indicators (e.g
antibiotics and drugs), which may be contained in other samples of
biological origin such as cerebrospinal fluid, urine, sweat, tears,
semen and saliva or derived therefrom.
[0196] Equally, some aspects of the above description have been
described with specific reference to electrochemical detection
systems, but the principles described are valid for other detection
means.
* * * * *