U.S. patent application number 12/574469 was filed with the patent office on 2010-01-28 for method of reducing interferences in an electrochemical sensor using two different applied potentials.
This patent application is currently assigned to LifeScan Scotland Ltd.. Invention is credited to Oliver William Hardwicke DAVIES.
Application Number | 20100018878 12/574469 |
Document ID | / |
Family ID | 34577659 |
Filed Date | 2010-01-28 |
United States Patent
Application |
20100018878 |
Kind Code |
A1 |
DAVIES; Oliver William
Hardwicke |
January 28, 2010 |
METHOD OF REDUCING INTERFERENCES IN AN ELECTROCHEMICAL SENSOR USING
TWO DIFFERENT APPLIED POTENTIALS
Abstract
The present invention is directed to a method of reducing the
effects of interfering compounds in the measurement of analytes and
more particularly to a method of reducing the effects of
interfering compounds in a system wherein the test strip utilizes
two or more working electrodes. In one embodiment of the present
invention, a first potential is applied to a first working
electrode and a second potential, having the same polarity but a
greater magnitude than the first potential, is applied to a second
working electrode.
Inventors: |
DAVIES; Oliver William
Hardwicke; (Croy, GB) |
Correspondence
Address: |
PHILIP S. JOHNSON;JOHNSON & JOHNSON
ONE JOHNSON & JOHNSON PLAZA
NEW BRUNSWICK
NJ
08933-7003
US
|
Assignee: |
LifeScan Scotland Ltd.
Inverness-shire
GB
|
Family ID: |
34577659 |
Appl. No.: |
12/574469 |
Filed: |
October 6, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10977154 |
Oct 29, 2004 |
7618522 |
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12574469 |
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60516252 |
Oct 31, 2003 |
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60558728 |
Mar 31, 2004 |
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60558424 |
Mar 31, 2004 |
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Current U.S.
Class: |
205/782 |
Current CPC
Class: |
A61B 5/150358 20130101;
A61B 5/150435 20130101; C12Q 1/001 20130101; A61B 5/150282
20130101; Y02A 90/10 20180101; A61B 5/14532 20130101; A61B 5/1486
20130101; A61B 5/150503 20130101; A61B 5/150022 20130101; G01N
27/3274 20130101; C12Q 1/006 20130101 |
Class at
Publication: |
205/782 |
International
Class: |
C12Q 1/54 20060101
C12Q001/54; G01N 27/26 20060101 G01N027/26 |
Claims
1. A method of reducing interferences in an electrochemical sensor
comprising: applying a first potential to a first working
electrode; applying a second potential to a second working
electrode, wherein said second potential is greater than the
absolute value of said first potential; measuring a first current,
at said first working electrode, which comprises an analyte current
and an interfering compound current; measuring a second current, at
said second working electrode, which comprises an analyte
overpotential current and an interfering compound overpotential
current, wherein said analyte overpotential current has a first
directly proportional relationship to said analyte current and,
wherein said interfering compound overpotential current has a
second directly proportional relationship to said interfering
compound current; and calculating a corrected current value
representative of an analyte concentration using an equation which
is a function of said first current, said second current, said
first directly proportional relationship, and said second directly
proportional relationship.
2. The method of claim 1, wherein said equation is A 1 = W 2 - YW 1
X - Y ##EQU00008## where A.sub.1 is said analyte current, W.sub.1
is said first current, W.sub.2 is said second current, X is an
analyte voltage effect factor, and Y is an interfering compound
voltage effect factor.
3. The method of claim 1, wherein said analyte is glucose.
4. The method of claim 1, wherein said first potential is between
about 385 millivolts and about 415 millivolts for said
electrochemical sensor which comprises a carbon working electrode
and a ferrocyanide redox mediator.
5. The method of claim 1, wherein said second potential is between
about 420 millivolts and about 1000 millivolts for said
electrochemical sensor which comprises a carbon working electrode
and a ferrocyanide redox mediator.
6. The method of claim 1, wherein said interfering compound current
results from the oxidation of at least one chemical chosen from the
group consisting of acetaminophen, ascorbic acid, bilirubin,
dopamine, gentisic acid, glutathione, levodopa, methyldopa,
tolazimide, tolbutamide, and uric acid.
7. (canceled)
8. (canceled)
Description
PRIORITY
[0001] This application claims priority from Provisional
Application No. 60/516,252 filed Oct. 31, 2003, Provisional
Application No. 60/558,728 filed Mar. 31, 2004, and Provisional
Application No. 60/558,427 filed Mar. 31, 2004, which are
incorporated herein by reference and to which we claim
priority.
RELATED APPLICATIONS
[0002] The present invention is related to the following co-pending
US Applications: U.S. patent application Ser. No. ______ [Attorney
Docket Number DDI-5042], filed on Oct. 29, 2004; U.S. patent
application Ser. No. ______ [Attorney Docket Number DDI-5064],
filed on Oct. 29, 2004; U.S. patent application Ser. No. ______
[Attorney Docket Number DDI-5065], filed on Oct. 29, 2004; U.S.
patent application Ser. No. ______ [Attorney Docket Number
DDI-5066], filed on Oct. 29, 2004; and U.S. patent application Ser.
No. ______ [Attorney Docket Number DDI-5067], filed on Oct. 29,
2004.
BACKGROUND OF INVENTION
[0003] Electrochemical glucose test strips, such as those used in
the OneTouch.RTM. Ultra.RTM. whole blood testing kit, which is
available from LifeScan, Inc., are designed to measure the
concentration of glucose in a blood sample from patients with
diabetes. The measurement of glucose is based upon the specific
oxidation of glucose by the flavo-enzyme glucose oxidase. During
this reaction, the enzyme becomes reduced. The enzyme is
re-oxidised by reaction with the mediator ferricyanide, which is
itself reduced during the course or the reaction. These reactions
are summarized below.
D-Glucose+GOx.sub.(ox).fwdarw.Gluconic acid+GOx.sub.(RED)
GOx.sub.(RED)+2Fe(CN).sub.6.sup.3-.fwdarw.GOx.sub.(OX)+2Fe(CN).sub.6.sup-
.4-
[0004] When the reaction set forth above is conducted with an
applied potential between two electrodes, an electrical current may
be created by the electrochemical re-oxidation of the reduced
mediator ion (ferrocyanide) at the electrode surface. Thus, since,
in an ideal environment, the amount of ferrocyanide created during
the chemical reaction described above is directly proportional to
the amount of glucose in the sample positioned between the
electrodes, the current generated would be proportional to the
glucose content of the sample. A redox mediator, such as
ferricyanide is a compound that exchanges electrons between a redox
enzyme such as glucose oxidase and an electrode. As the
concentration of glucose in the sample increases, the amount of
reduced mediator formed also increases, hence, there is a direct
relationship between current resulting from the re-oxidation of
reduced mediator and glucose concentration. In particular, the
transfer of electrons across the electrical interface results in a
flow of current (2 moles of electrons for every mole of glucose
that is oxidized). The current resulting from the introduction of
glucose may, therefore, be referred to as the glucose current.
[0005] Because it can be very important to know the concentration
of glucose in blood, particularly in people with Diabetes, meters
have been developed using the principals set forth above to enable
the average person to sample and test their blood to determine the
glucose concentration at any given time. The Glucose Current
generated is monitored by the meter and converted into a reading of
glucose concentration using a preset algorithm that relates current
to glucose concentration via a simple mathematical formula. In
general, the meters work in conjunction with a disposable strip
that includes a sample chamber and at least two electrodes disposed
within the sample chamber in addition to the enzyme (e.g. glucose
oxidase) and mediator (e.g. ferricyanide). In use, the user pricks
their finger or other convenient site to induce bleeding and
introduces a blood sample to the sample chamber, thus starting the
chemical reaction set forth above.
[0006] In electrochemical terms, the function of the meter is two
fold. Firstly, it provides a polarizing voltage (approximately 0.4
V in the case of OneTouch.RTM. Ultra.RTM.) that polarizes the
electrical interface and allows current flow at the carbon working
electrode surface. Secondly, it measures the current that flows in
the external circuit between the anode (working electrode) and the
cathode (reference electrode). The meter may, therefore be
considered to be a simple electrochemical system that operates in
two-electrode mode although, in practice, third and, even fourth
electrodes may be used to facilitate the measurement of glucose
and/or perform other functions in the meter.
[0007] In most situations, the equation set forth above is
considered to be a sufficient approximation of the chemical
reaction taking place on the test strip and the meter reading a
sufficiently accurate representation of the glucose content of the
blood sample. However, under certain circumstances and for certain
purposes, it may be advantageous to improve the accuracy of the
measurement. For example, where a portion of the current measured
at the electrode results from the presence of other chemicals or
compounds in the sample. Where such additional chemicals or
compounds are present, they may be referred to as interfering
compounds and the resulting additional current may be referred to
as Interfering Currents
[0008] Examples of potentially interfering chemicals (i.e.
compounds found in physiological fluids such as blood that may
generate Interfering Currents in the presence of an electrical
field) include ascorbate, urate and acetaminophen (Tylenol.TM. or
Paracetamol). One mechanism for generating Interfering Currents in
an electrochemical meter for measuring the concentration of an
analyte in a physiological fluid (e.g. a glucose meter) involves
the oxidation of one or more interfering compounds by reduction of
the enzyme (e.g. glucose oxidase). A further mechanism for
generating Interfering Currents in such a meter involves the
oxidation of one or more interfering compounds by reduction of the
mediator (e.g. ferricyanide). A further mechanism for generating
Interfering Currents in such a meter involves the oxidation of one
or more interfering compounds at the working electrode. Thus, the
total current measured at the working electrode is the
superposition of the current generated by oxidation of the analyte
and the current generated by oxidation of interfering compounds.
Oxidation of interfering compounds may be a result of interaction
with the enzyme, the mediator or may occur directly at the working
electrode.
[0009] In general, potentially interfering compounds can be
oxidized at the electrode surface and/or by a redox mediator. This
oxidation of the interfering compound in a glucose measurement
system causes the measured oxidation current to be dependent on
both the glucose and the interfering compound. Therefore, if the
concentration of interfering compound oxidizes as efficiently as
glucose and/or the interfering compound concentration is
significantly high relative to the glucose concentration, it may
impact the measured glucose concentration.
[0010] The co-oxidization of analyte (e.g. glucose) with
interfering compounds is especially problematic when the standard
potential (i.e. the potential at which a compound is oxidized) of
the interfering compound is similar in magnitude to the standard
potential of the redox mediator, resulting in a significant portion
of the Interference Current being generated by oxidation of the
interfering compounds at the working electrode. Electrical current
resulting from the oxidation of interfering compounds at the
working electrode may be referred to as direct interference
current. It would, therefore, be advantageous to reduce or minimize
the effect of the direct interference current on the measurement of
analyte concentration. Previous methods of reducing or eliminating
direct interference current include designing test strips that
prevent the interfering compounds from reaching the working
electrode, thus reducing or eliminating the direct interference
current attributable to the excluded compounds.
[0011] One strategy for reducing the effects of interfering
compounds that generate Direct interference current is to place a
negatively charged membrane on top of the working electrode. As one
example, a sulfonated fluoropolymer such as NAFION.TM. may be
placed over the working electrode to repel all negatively charged
chemicals. In general, many interfering compounds, including
ascorbate and urate, have a negative charge, and thus, are excluded
from being oxidized at the working electrode when the surface of
the working electrode is covered by a negatively charged membrane.
However, because some interfering compounds, such as acetaminophen,
are not negatively charged, and thus, can pass through the
negatively charged membrane, the use of a negatively charged
membrane will not eliminate the Direct interference current.
Another disadvantage of covering the working electrode with a
negatively charged membrane is that commonly used redox mediators,
such as ferricyanide, are negatively charged and cannot pass
through the membrane to exchange electrons with the electrode. A
further disadvantage of using a negatively charged membrane over
the working electrode is the potential to slow the diffusion of the
reduced mediator to the working electrode, thus increasing the test
time. A further disadvantage of using a negatively charged membrane
over the working electrode is the increased complexity and expense
of manufacturing the test strips with a negatively charged
membrane.
[0012] Another strategy that can be used to decrease the effects of
Direct Interfering Currents is to position a size selective
membrane on top of the working electrode. As one example, a 100
Dalton size exclusion membrane such as cellulose acetate may be
placed over the working electrode to exclude compounds having a
molecular weight greater than 100 Daltons. In this embodiment, the
redox enzyme such as glucose oxidase is positioned over the size
exclusion membrane. Glucose oxidase generates hydrogen peroxide, in
the presence of glucose and oxygen, in an amount proportional to
the glucose concentration. It should be noted that glucose and most
redox mediators have a molecular weight greater than 100 Daltons,
and thus, cannot pass through the size selective membrane. Hydrogen
peroxide, however, has a molecular weight of 34 Daltons, and thus,
can pass through the size selective membrane. In general, most
interfering compounds have a molecular weight greater than 100
Daltons, and thus, are excluded from being oxidized at the
electrode surface. Since some interfering compounds have smaller
molecular weights, and thus, can pass through the size selective
membrane, the use of a size selective membrane will not eliminate
the Direct interference current. A further disadvantage of using a
size selective membrane over the working electrode is the increased
complexity and expense of manufacturing the test strips with a size
selective membrane.
[0013] Another strategy that can be used to decrease the effects of
Direct interference current is to use a redox mediator with a low
redox potential, for example, a redox potential of between about
-300 mV to +100 mV (vs a saturated calomel electrode). This allows
the applied potential to the working electrode to be relatively low
which, in turn, decreases the rate at which interfering compounds
are oxidized by the working electrode. Examples of redox mediators
having a relatively low redox potential include osmium bipyridyl
complexes, ferrocene derivatives, and quinone derivatives. However,
redox mediators having a relatively low potential are often
difficult to synthesize, relatively unstable and relatively
insoluble.
[0014] Another strategy that can be used to decrease the effects of
interfering compounds is to use a dummy electrode in conjunction
with the working electrode. The current measured at the dummy
electrode may then be subtracted from the current measured at the
working electrode in order to compensate for the effect of the
interfering compounds. If the dummy electrode is bare (i.e. not
covered by an enzyme or mediator), then the current measured at the
dummy electrode will be proportional to the Direct interference
current and subtracting the current measured at the dummy electrode
from the current measured at the working electrode will reduce or
eliminate the effect of the direct oxidation of interfering
compounds at the working electrode. If the dummy electrode is
coated with a redox mediator then the current measured at the dummy
electrode will be a combination of Direct interference current and
interference current resulting from reduction of the redox mediator
by an interfering compound. Thus, subtracting the current measured
at the dummy electrode coated with a redox mediator from the
current measured at the working electrode will reduce or eliminate
the effect of the direct oxidation of interfering compounds and the
effect of interference resulting from reduction of the redox
mediator by an interfering compound at the working electrode. In
some instances the dummy electrode may also be coated with an inert
protein or deactivated redox enzyme in order to simulate the effect
of the redox mediator and enzyme on diffusion. Because it is
preferable that test strips have a small sample chamber so that
people with diabetes do not have to express a large blood sample,
it may not be advantageous to include an extra electrode which
incrementally increases the sample chamber volume where the extra
electrode is not used to measure the analyte (e.g. glucose).
Further, it may be difficult to directly correlate the current
measured at the dummy electrode to interference currents at the
working electrode. Finally, since the dummy electrode may be coated
with a material or materials (e.g. redox mediator) which differ
from the materials used to cover the working electrode (e.g. redox
mediator and enzyme), test strips which use dummy electrodes as a
method of reducing or eliminating the effect of interfering
compounds in a multiple working electrode system may increase the
cost and complexity of manufacturing the test strip.
[0015] Certain test strip designs which utilize multiple working
electrodes to measure analyte, such as the system used in the
OneTouch.RTM. Ultra.RTM. glucose measurement system are
advantageous because the use of two working electrodes. In such
systems, it would, therefore, be advantageous to develop a method
of reducing or eliminating the effect of interfering compounds.
More particularly, it would be advantageous to develop a method of
reducing or eliminating the effect of interfering compounds without
utilizing a dummy electrode, an intermediate membrane or a redox
mediator with a low redox potential.
SUMMARY OF INVENTION
[0016] The present invention is directed to a method of reducing
the effects of interfering compounds in the measurement of analytes
and more particularly to a method of reducing the effects of
interfering compounds in a system wherein the test strip utilizes
two or more working electrodes. In one embodiment of the present
invention, a first potential is applied to a first working
electrode and a second potential, having the same polarity but a
greater magnitude than the first potential, is applied to a second
working electrode. The magnitude of the second potential may also
be less than the first potential for an embodiment where a
reduction current is used to measure the analyte concentration. In
one embodiment, the first working electrode and second working
electrode may be covered with an enzyme reagent and redox mediator
that are analyte specific. The first potential applied to the first
working electrode is selected such that it is sufficient to oxidize
reduced redox mediator in a diffusion limited manner while the
second potential is selected to have a magnitude (i.e. absolute
value) greater than the magnitude of the first potential, resulting
in a more efficient oxidation of at the second working electrode.
In this embodiment of the invention, the current measured at the
first working electrode includes an analyte current and interfering
compound current while the current measured at the second working
electrode includes an analyte overpotential current and an
interfering compound overpotential current. It should be noted that
the analyte current and the analyte overpotential current both
refer to a current that corresponds to the analyte concentration
and that the current is a result of a reduced mediator oxidation.
In an embodiment of this invention, the relationship between the
currents at the first working electrode and second working
electrode may be defined by the following equation,
A 1 = W 2 - YW 1 X - Y ##EQU00001##
where A.sub.1 is the analyte current at the first working
electrode, W.sub.1 is the current measured at the first working
electrode, W.sub.2 is the current measured at the second working
electrode, X is an analyte dependent voltage effect factor and Y is
an interfering compound dependent voltage effect factor. Using the
equation set forth above, in a method according to the present
invention, it is possible to reduce the effect of oxidation
currents resulting from the presence of interfering compounds and
calculate a corrected current value that is more representative of
the concentration of analyte in the sample being measured.
[0017] In one embodiment of the present invention, the
concentration of glucose in a sample placed on a test strip can be
calculated by placing the sample on a test strip having a first
working electrode and second working electrode and a reference
electrode, at least the first working electrode and second working
electrodes being coated with chemical compounds (e.g. an enzyme and
a redox mediator) adapted to facilitate the oxidation of glucose
and the transfer of electrons from the oxidized glucose to the
first working electrode and the second working electrode when a
potential is applied between the first working electrode and the
reference electrode, and the second working electrode and the
reference electrode. In accordance with the present invention, a
first potential is applied between the first working electrode and
the reference electrode, the first potential being selected to have
a magnitude sufficient to ensure that the magnitude of the current
generated by oxidation of the glucose in the sample is limited only
by factors other than applied voltage (e.g. diffusion). In
accordance with the present invention, a second potential is
applied between the second working electrode and the reference
electrode, the second potential being greater in magnitude than the
first potential and, in one embodiment of the present invention,
the second potential being selected to increase the oxidation of
interfering compounds at the second working electrode. In a further
embodiment of the present invention, the following equation may be
used to reduce the effect of oxidation current resulting from the
presence of interfering compounds on the current used to calculate
the concentration of glucose in the sample. In particular, the
glucose concentration may be derived using a calculated current
A.sub.1G where:
A 1 G = W 2 - YW 1 X G - Y ##EQU00002##
where A.sub.1G is a glucose current, W.sub.1 is the current
measured at the first working electrode, W.sub.2 is the current
measured at the second working electrode, X.sub.G is a glucose
dependent voltage effect factor and Y is an interfering compound
dependent voltage effect factor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The novel features of the invention are set forth with
particularity in the appended claims. A better understanding of the
features and advantages of the present invention will be obtained
by reference to the following detailed description that sets forth
illustrative embodiments, in which the principles of the invention
are utilized, and the accompanying drawings of which:
[0019] FIG. 1 is an exploded perspective view of a test strip
embodiment for use in the present invention.
[0020] FIG. 2 is a schematic view of a meter and strip for use in
the present invention.
[0021] FIG. 3 is a hydrodynamic voltammogram illustrating the
dependence of applied voltage with measured current.
DETAILED DESCRIPTION OF THE INVENTION
[0022] While the present invention is particularly adapted to the
measurement of glucose concentration in blood, it will be apparent
to those of skill in the art that the method described herein may
be adapted to improve the selectivity of other systems used for the
electrochemical measurement of analytes in physiological fluids.
Examples of systems that may be adapted to improve selectivity
using the method according to the present invention include
electrochemical sensors used to measure the concentration of
lactate, lactate, alcohol, cholesterol, amino acids, choline, and
fructosamine in physiological fluids. Examples of physiological
fluids that may contain such analytes include blood, plasma, serum,
urine, and interstitial fluid. It will further be understood that,
while the method of the present invention is described in an
electrochemical system where the measured current is produced by
oxidation, the invention would be equally applicable to a system
wherein the measured current is produced by reduction.
[0023] The present invention is directed to a method for improving
the selectivity of an electrochemical measuring system that is
particularly adapted for use in a blood glucose measurement system.
More particularly, the present invention is directed to a method
for improving the selectivity of a blood glucose measurement system
by partially or wholly correcting for the effect of the direct
interference current. Selectivity in such systems being a measure
of the ability of the system to accurately measure the glucose
concentration in a sample of physiological fluid which includes one
or more compounds which create an interfering current. Improvement
of selectivity thus reduces the current generated at the working
electrode by the presence of interfering compounds (i.e. compounds
other than glucose which oxidize to generate interfering current)
and makes the measured current more representative of the glucose
concentration. In particular, the measured current may be a
function of the oxidation of interfering compounds commonly found
in physiological fluids such as, for example, acetaminophen
(Tylenol.TM. or Paracetamol), ascorbic acid, bilirubin, dopamine,
gentisic acid, glutathione, levodopa, methyldopa, tolazimide,
tolbutamide and uric acid. Such interfering compounds may be
oxidized by, for example, reacting chemically with the redox
mediator or by oxidizing at the electrode surface.
[0024] In a perfectly selective system, there would be no oxidation
current generated by any interfering compound and the entire
oxidation current would be generated by oxidation of glucose.
However, if oxidation of interfering compounds and the resulting
oxidation current cannot be avoided the present invention describes
a method of removing some or all of the effect of interfering
compounds by quantifying the proportion of the overall oxidation
current generated by the interfering compounds and subtracting that
quantity from the overall oxidation current. In particular, in a
method according to the present invention, using a test strip that
includes first working electrode and second working electrode, two
different potentials are applied and the oxidation current
generated at each of the working electrodes is measure used to
estimate the respective oxidation current proportions for both the
glucose and interfering compounds.
[0025] In one embodiment of a method according to the present
invention, a test strip is used which includes a sample chamber
containing a first working electrode, a second working electrode,
and a reference electrode. The first working electrode, the second
working electrode and the reference electrodes are covered by
glucose oxidase (the enzyme) and a Ferricyanide (the redox
mediator). When a sample of blood (the physiological fluid) is
placed in the sample chamber, the glucose oxidase is reduced by
glucose in the blood sample generating gluconic acid. The gluconic
acid is then oxidized by reduction of the Ferricyanide to
Ferrocyanide, yielding a reduced redox mediator with a
concentration proportional to the glucose concentration. An example
of a test strip that may be suitable for use in a method according
to the present invention is the OneTouch.RTM. Ultra.RTM. test strip
sold by LifeScan, Inc. of Milpitas, Calif. Other suitable strips
are described in international publication WO 01/67099A1 and WO
01/73124A2.
[0026] In one embodiment of a method according to the present
invention a first potential is applied to a first working electrode
and a second potential is applied to the second working electrode.
In this embodiment, the first potential is selected to be in a
range in which the glucose current response is relatively
insensitive to the applied potential and thus the magnitude of the
glucose current at the first working electrode is limited by the
amount of reduced redox mediator diffusing to the first working
electrode. It should be noted that glucose is not directly oxidized
at a working electrode, but instead is indirectly oxidized through
using a redox enzyme and a redox mediator. In the description of
the present invention, the glucose current refers to an oxidation
of reduced redox mediator that correlates to the glucose
concentration. In an embodiment of the present invention where
ferri/ferrocyanide is the redox mediator and carbon is the working
electrode, the first potential may range from about 0 millivolts to
about 500 millivolts, and more preferably from about 385 millivolts
to about 415 millivolts, and yet even more preferably may range
from about 395 to 405 mV. A second potential is applied to a second
working electrode such that the second potential is greater than
the first potential. Where the applied potential is greater than
the potential needed to oxidize the glucose. In an embodiment of
the present invention where ferri/ferrocyanide is the redox
mediator and carbon is the working electrode, the second potential
may range from about 50 millivolts to about 1000 millivolts, and
more preferably from about 420 millivolts to about 1000
millivolts.
[0027] Because the glucose current does not increase or increases
only minimally with increasing potential, the glucose current at
the second working electrode should be substantially equal to the
glucose current at the first working electrode, even though the
potential at the second working electrode is greater than the
potential at the first electrode. Thus, any additional current
measured at the second working electrode may be attributed to the
oxidation of interfering compounds. In other words, the higher
potential at the second working electrode should cause a glucose
overpotenital current to be measured at the second working
electrode which is equal or substantially equal in magnitude to the
glucose current at the first working electrode because the first
potential and second potential are in a limiting glucose current
range which is insensitive to changes in applied potential.
However, in practice, other parameters may have an impact on the
measured current, for example, where a higher potential is applied
to the second working electrode, there is often a slight increase
in the overall current at the second working electrode as a result
of an IR drop or capacitive effects. When an IR drop (i.e.
uncompensated resistance) is present in the system, a higher
applied potential causes an increase in the measured current
magnitude. Examples of IR drops may be the nominal resistance of
the first working electrode, second working electrode, the
reference electrode, the physiological fluid between the working
electrode and the reference electrode. In addition, the application
of a higher potential results in the formation of a larger ionic
double layer which forms at the electrode/liquid interface,
increasing the ionic capacitance and the resulting current at
either the first working electrode or second working electrode.
[0028] In order to determine the actual relationship between the
glucose current measured at the first working electrode and the
second working electrode, it is necessary to develop a suitable
equation. It should be noted that the glucose current at the second
working electrode may also be referred to as a glucose
overpotential current. A directly proportional relationship between
the glucose current and the glucose overpotential current may be
described by the following equation.
X.sub.G.times.A.sub.1G=A.sub.2G (eq 1)
where X.sub.G is a glucose dependent voltage effect factor,
A.sub.1G is the glucose current at the first working electrode and
A.sub.2G is the glucose current at the second working
electrode.
[0029] In an embodiment of the present invention, where
ferri/ferrocyanide is the redox mediator and carbon is the working
electrode, the voltage effect factor X.sub.G for glucose may be
expected to be between about 0.95 any about 1.1. In this embodiment
of the invention, higher potentials do not have a significant
impact on the glucose oxidation current because the redox mediator
(ferrocyanide) has fast electron transfer kinetics and reversible
electron transfer characteristics with the working electrode.
Because the glucose current does not increase with increasing
potential after a certain point, the glucose current may be said to
be saturated or in a diffusion limited regime.
[0030] In the embodiment of the present invention described above,
glucose is indirectly measured by oxidizing ferrocyanide at the
working electrode and where the ferrocyanide concentration is
directly proportional to the glucose concentration. The standard
potential (E.sup.o) value for a particular electrochemical compound
is a measure of that compound's ability to exchange electrons with
other chemical compounds. In the method according to the present
invention, the potential at the first working electrode is selected
to be greater than the standard potential (E.sup.o) of the redox
mediator. Because the first potential is selected such that it is
sufficiently greater than the E.sup.o value of the redox couple,
the oxidation rate does not increase substantially as the applied
potential increases. Thus, applying a greater potential at the
second working electrode will not increase the oxidation at the
second working electrode and any increased current measured at the
higher potential electrode must be due to other factors, such as,
for example, oxidation of interfering compounds.
[0031] FIG. 3 is a hydrodynamic voltammogram illustrating the
dependence of applied voltage with measured current where
ferri/ferrocyanide is the redox mediator and carbon is the working
electrode. Each data point on the graph represents at least one
experiment where a current is measured 5 seconds after applying a
voltage across a working electrode and a reference electrode. FIG.
3 shows that the current forms an onset of a plateau region at
about 400 mV because the applied voltage is sufficiently greater
than of the E.sup.o value of ferrocyanide. Thus, as illustrated in
FIG. 3, as the potential reaches approximately 400 mV, the glucose
current becomes saturated because the oxidation of ferrocyanide is
diffusion limited (i.e. the diffusion of ferrocyanide to the
working electrode limits the magnitude of the measured current and
is not limited by the electron transfer rate between ferrocyanide
and the electrode).
[0032] In general, current generated by the oxidation of
interfering compounds is not saturated by increases in applied
voltage and shows a much stronger dependence on applied potential
than current generated by oxidation of ferrocyanide (the
ferrocyanide having been generated from the interaction of glucose
with the enzyme and the enzyme with ferrocyanide. Typically,
interfering compounds have slower electron transfer kinetics than
redox mediators (i.e. ferrocyanide). This difference is ascribed to
the fact that most interfering compounds undergo an inner sphere
electron transfer pathway as opposed to the faster outer sphere
electron transfer pathway of ferrocyanide. A typical inner sphere
electron transfer requires a chemical reaction to occur, such as a
hydride transfer, before transferring an electron. In contrast, an
outer sphere electron transfer does not require a chemical reaction
before transferring an electron. Therefore, inner sphere electron
transfer rates are typically slower than outer sphere electron
transfers because they require an additional chemical reaction
step. The oxidation of ascorbate to dehydroascorbate is an example
of an inner sphere oxidation that requires the liberation of two
hydride moieties. The oxidation of ferrocyanide to ferricyanide is
an example of an outer sphere electron transfer. Therefore, the
current generated by interfering compounds generally increases when
testing at a higher potential.
[0033] A relationship between an interfering compound current at
the first working electrode and an interfering compound
overpotential current at the second working electrode can be
described by the following equation,
Y.times.I.sub.1=I.sub.2 (eq 2)
where Y is an interfering compound dependent voltage effect factor,
I.sub.1 is the interfering compound current, and I.sub.2 is the
interfering compound overpotential current. Because the interfering
compound voltage effect factor Y is dependent upon a number of
factors, including, the particular interfering compound or
compounds of concern and the material used for the working
electrodes, calculation of a particular interfering compound
dependent voltage effect factor for a particular system, test
strip, analyte and interfering compound or compounds may require
experimentation to optimize the voltage effect factor for those
criteria. Alternatively, under certain circumstances, appropriate
voltage effect factors may be derived or described
mathematically.
[0034] In an embodiment of the present invention where
ferri/ferrocyanide is the redox mediator and carbon is the working
electrode, the interfering compound dependent voltage effect factor
Y could be mathematically described using the Tafel equation for
I.sub.1 and I.sub.2,
I 1 = a ' exp ( .eta. 1 b ' ) ( eq 2 a ) I 2 = a ' exp ( .eta. 2 b
' ) ( eq 2 b ) ##EQU00003##
where .eta..sub.1=E.sub.1-E.sup.o, .eta..sub.2=E.sub.2-E.sup.o, b'
is a constant depending of the specific electroactive interfering
compound, E.sub.1 is the first potential, and E.sub.2 is the second
potential. The value of E.sup.o (the standard potential of a
specific interfering compound) is not important because it is
canceled out in the calculation of .DELTA..eta.. Equations 2, 2a,
2b can be combined and rearranged to yield the following
equation,
Y = exp ( .DELTA. .eta. b ' ) ( eq 2 c ) ##EQU00004##
where .DELTA..eta.=E.sub.1-E.sub.2. Equation 2c provides a
mathematical relationship describing the relationship between
.DELTA..eta. (i.e. the difference between the first potential and
the second potential) and the interfering compound dependent
voltage effect factor Y. In an embodiment of the present invention,
Y may range from about 1 to about 100, and more preferably between
about 1 and 10. In an embodiment of this invention, the interfering
compound dependent voltage effect factor Y may be determined
experimentally for a specific interfering compound or combination
of interfering compounds. It should be noted that the interfering
compound dependent voltage effect factor Y for interfering
compounds is usually greater than voltage effect factor X.sub.G for
glucose. As the following sections will describe, the mathematical
relationship of a) the interfering compound current I.sub.1 and the
interfering compound overpotential current I.sub.2; and b) the
glucose current A.sub.1G and the glucose overpotential current
A.sub.2G will allow a glucose algorithm to be proposed which will
reduce the effects of interfering compounds for measuring
glucose.
[0035] In an embodiment of the present invention, an algorithm was
developed to calculate a corrected glucose current (i.e. A.sub.1G
and A.sub.2G) which is independent of interferences. After dosing a
sample onto a test strip, a first potential is applied to the first
working electrode and a second potential is applied to the second
working electrode. At the first working electrode, a first current
is measured which can be described by the following equation,
W.sub.1=A.sub.1G+I.sub.1 (eq 3)
where W.sub.1 is the first current at the first working electrode.
In other words, the first current includes a superposition of the
glucose current A.sub.1G and the interfering compound current
I.sub.1. More specifically, the interfering compound current may be
a direct interfering current which has been described hereinabove.
At the second working electrode, a second current is measured at
the second potential or overpotential which can be described by the
following equation,
W.sub.2=A.sub.2G+I.sub.2 (eq 4)
where W.sub.2 is the second current at the second working
electrode, A.sub.2G is the glucose overpotential current measured
at the second potential, and I.sub.2 is the interfering compound
overpotential current measured at the second potential. More
specifically, the interfering compound overpotential current may be
a Direct Interfering compound Current which has been described
hereinabove. Using the previously described 4 equations (eq's 1 to
4) which contain 4 unknowns (A.sub.1G, A.sub.2G, I.sub.1, and
I.sub.2), it is now possible to calculate a corrected glucose
current equation which is independent of interfering compounds.
[0036] As the first step in the derivation, A.sub.2G from eq 1 and
I.sub.2 from eq 2 can be substituted into eq 4 to give the
following eq 5.
W.sub.2=X.sub.GA.sub.1G+YI.sub.1 (eq 5)
[0037] Next, eq 3 is multiplied by interfering compound dependent
voltage effect factor Y for interfering compounds to give eq 6.
YW.sub.1=YA.sub.1G+YI.sub.1 (eq 6)
Eq 5 can now be subtracted from eq 6 to give the following form
shown in eq 7
W.sub.2-YW.sub.1=X.sub.GA.sub.1G-YA.sub.1G (eq 7)
Eq 7 can now be rearranged to solve for the corrected glucose
current A.sub.1G measured at the first potential as shown in eq
8.
A 1 G = W 2 - YW 1 X G - Y ( eq 8 ) ##EQU00005##
Eq 8 outputs a corrected glucose current A.sub.1G which removes the
effects of interferences requiring only the current output of the
first working electrode and second working electrode (e.g. W.sub.1
and W.sub.2), glucose dependent voltage effect factors X.sub.G, and
interfering compound dependent voltage effect factor Y for
interfering compounds.
[0038] A glucose meter containing electronics is electrically
interfaced with a glucose test strip to measure the current from
W.sub.1 and W.sub.2. In one embodiment of the present invention,
X.sub.G and Y may be programmed into the glucose meter as read only
memory. In another embodiment of the present invention X.sub.G and
Y may be transferred to the meter via a calibration code chip. The
calibration code chip would have in its memory a particular set of
values for X.sub.G and Y which would be calibrated for a particular
lot of test strips. This would account for test strip lot-to-lot
variations that may occur in X.sub.G and Y.
[0039] In another embodiment of the present invention, the
corrected glucose current in eq 8 may be used by the meter only
when a certain threshold is exceeded. For example, if W.sub.2 is
about 10% or greater than W.sub.1, then the meter would use eq 8 to
correct for the current output. However, if W.sub.2 is about 10% or
less than W.sub.1, the interfering compound concentration is low
and thus the meter can simply take an average current value between
W.sub.1 and W.sub.2 to improve the accuracy and precision of the
measurement. Instead of simply averaging the current of W.sub.1 and
W.sub.2, a more accurate approach may be to average W.sub.1
with
W 2 X G ##EQU00006##
where the glucose dependent voltage effect factor X.sub.G is taken
into account (note
W 2 X G ##EQU00007##
approximately equals A.sub.1G according to eq 1 and 4 when I.sub.2
is low). The strategy of using eq 8 only under certain situations
where it is likely that a significant level of interferences are in
the sample mitigates the risk of overcorrecting the measured
glucose current. It should be noted that when W.sub.2 is
sufficiently greater than W.sub.1 by a large amount (e.g. about
100% or more), this is an indicator of having an unusually high
concentration of interferences. In such a case, it may be desirable
to output an error message instead of a glucose value because a
very high level of interfering compounds may cause a breakdown in
the accuracy of eq 8.
[0040] The following sections will describe a possible test strip
embodiment which may be used with the proposed algorithm of the
present invention as shown in eq 8. FIG. 1 is an exploded
perspective view of test strip 600, which includes six layers
disposed upon a base substrate 5. These six layers are a conductive
layer 50, an insulation layer 16, a reagent layer 22, an adhesive
layer 60, a hydrophilic layer 70, and a top layer 80. Test strip
600 may be manufactured in a series of steps wherein the conductive
layer 50, insulation layer 16, reagent layer 22, adhesive layer 60
are deposited on base substrate 5 using, for example, a screen
printing process. Hydrophilic layer 70 and top layer 80 may be
deposed from a roll stock and laminated onto base substrate 5. The
fully assembled test strip forms a sample receiving chamber that
can accept a blood sample so that it can be analyzed.
[0041] Conductive layer 50 includes reference electrode 10, first
working electrode 12, second working electrode 14, a first contact
13, a second contact 15, a reference contact 11, and a strip
detection bar 17. Suitable materials which may be used for the
conductive layer are Au, Pd, Ir, Pt, Rh, stainless steel, doped tin
oxide, carbon, and the like. Preferably, the material for the
conductive layer may be a carbon ink such as those described in
U.S. Pat. No. 5,653,918.
[0042] Insulation layer 16 includes cutout 18 which exposes a
portion of reference electrode 10, first working electrode 12, and
second working electrode 14 which can be wetted by a liquid sample.
As a non-limiting example, insulation layer (16 or 160) may be
Ercon E6110-116 Jet Black Insulayer Ink which may be purchased from
Ercon, Inc.
[0043] Reagent layer 22 may be disposed on a portion of conductive
layer 50 and insulation layer 16. In an embodiment of the present
invention, reagent layer 22 may include chemicals such as a redox
enzyme and redox mediator which selectivity react with glucose.
During this reaction, a proportional amount of a reduced redox
mediator can be generated that then can be measured
electrochemically so that a glucose concentration can be
calculated. Examples of reagent formulations or inks suitable for
use in the present invention can be found in U.S. Pat. Nos.
5,708,247 and 6,046,051; published international applications
WO01/67099 and WO01/73124, all of which are incorporated by
reference herein.
[0044] Adhesive layer 60 includes first adhesive pad 24, second
adhesive pad 26, and third adhesive pad 28. The side edges of first
adhesive pad 24 and second adhesive pad 26 located adjacent to
reagent layer 22 each define a wall of a sample receiving chamber.
In an embodiment of the present invention, the adhesive layer may
comprise a water based acrylic copolymer pressure sensitive
adhesive which is commercially available from Tape Specialties LTD
in Tring, Herts, United Kingdom (part#A6435).
[0045] Hydrophilic layer 70 includes a distal hydrophilic pad 32
and proximal hydrophilic pad 34. As a non-limiting example,
hydrophilic layer 70 be a polyester having one hydrophilic surface
such as an anti-fog coating which is commercially available from
3M. It should be noted that both distal hydrophilic film 32 and
proximal hydrophilic film 34 are visibly transparent enabling a
user to observe a liquid sample filling the sample receiving
chamber.
[0046] Top layer 80 includes a clear portion 36 and opaque portion
38. Top layer 80 is disposed on and adhered to hydrophilic layer
70. As a non-limiting example, top layer 40 may be a polyester. It
should be noted that the clear portion 36 substantially overlaps
proximal hydrophilic pad 32 which allows a user to visually confirm
that the sample receiving chamber is sufficiently filled. Opaque
portion 38 helps the user observe a high degree of contrast between
a colored fluid such as, for example, blood within the sample
receiving chamber and the opaque section of the top film.
[0047] FIG. 2 is a simplified schematic showing a meter 500
interfacing with a test strip 600. Meter 500 has three electrical
contacts that form an electrical connection to first working
electrode 12, second working electrode 14, and reference electrode
10. In particular connector 101 connects voltage source 103 to
first working electrode 12, connector 102 connects voltage source
104 to second working electrode 14 and common connector 100
connects voltage source 103 and 104 to reference electrode 10. When
performing a test, voltage source 103 in meter 500 applies a first
potential E.sub.1 between first working electrode 12 and reference
electrode 10 and voltage source 104 applies a second potential
E.sub.2 between second working electrode 14 and reference electrode
10. A sample of blood is applied such that first working electrode
12, second working electrode 14, and reference electrode 10 are
covered with blood. This causes reagent layer 22 to become hydrated
which generates ferrocyanide in an amount proportional to the
glucose and/or interfering compound concentration present in the
sample. After about 5 seconds from the sample application, meter
500 measures an oxidation current for both first working electrode
12 and second working electrode 14.
[0048] In the previously described first and second test strip
embodiments, the first working electrode 12 and second working
electrode 14 had the same area. It should be noted that the present
invention is not limited to test strips having equal areas. For
alternative embodiments to the previously described strips where
the areas are different, the current output for each working
electrode must be normalized for area. Because the current output
is directly proportional to area, the terms within Equation 1 to
Equation 8 may be in units of amperes (current) or in amperes per
unit area (i.e. current density).
[0049] It will be recognized that equivalent structures may be
substituted for the structures illustrated and described herein and
that the described embodiment of the invention is not the only
structure that may be employed to implement the claimed invention.
In addition, it should be understood that every structure described
above has a function and such structure can be referred to as a
means for performing that function. While preferred embodiments of
the present invention have been shown and described herein, it will
be obvious to those skilled in the art that such embodiments are
provided by way of example only. Numerous variations, changes, and
substitutions will now occur to hose skilled in the art without
departing from the invention. It should be understood that various
alternatives to the embodiments of the invention described herein
may be employed in practicing the invention. It is intended that
the following claims define the scope of the invention and that
methods and structures within the scope of these claims and their
equivalents be covered thereby.
* * * * *