U.S. patent application number 10/977086 was filed with the patent office on 2005-06-30 for method of reducing the effect of direct and mediated interference current in an electrochemical test strip.
Invention is credited to Baskeyfield, Damian Edward Haydon, Davies, Oliver William Hardwicke, Leiper, Elaine, Marshall, Robert, Whyte, Lynsey.
Application Number | 20050139489 10/977086 |
Document ID | / |
Family ID | 34577659 |
Filed Date | 2005-06-30 |
United States Patent
Application |
20050139489 |
Kind Code |
A1 |
Davies, Oliver William Hardwicke ;
et al. |
June 30, 2005 |
Method of reducing the effect of direct and mediated interference
current in an electrochemical test strip
Abstract
The present invention is directed to a method of reducing
interferences in an electrochemical sensor wherein the method
includes the step of measuring a first current at a first working
electrode, the first working electrode being covered by an active
reagent layer, the step of measuring a second current at a second
working electrode, the second working electrode being covered by an
inactive reagent layer and the step of calculating a corrected
current value representative of a glucose concentration using a
ratio of an active area of the first working electrode to an
inactive area of the second working electrode. The present
invention is further directed to a method of reducing interferences
in an electrochemical sensor wherein the method includes the step
of measuring a first current at a first working electrode, the
first working electrode being covered by an active reagent layer,
the step of measuring a second current at a second working
electrode, wherein the active reagent layer is disposed on an
active region of the second working electrode and an inactive
region of the second working electrode is covered by an inactive
reagent layer and the step of calculating a corrected current value
representative of a glucose concentration using a ratio of an
active region on the first and second working electrodes and an
inactive region on the second working electrode.
Inventors: |
Davies, Oliver William
Hardwicke; (Croy, GB) ; Marshall, Robert;
(Conon Bridge, GB) ; Baskeyfield, Damian Edward
Haydon; (Auldeam, GB) ; Whyte, Lynsey;
(Lochardil, GB) ; Leiper, Elaine; (Dores Road,
GB) |
Correspondence
Address: |
PHILIP S. JOHNSON
JOHNSON & JOHNSON
ONE JOHNSON & JOHNSON PLAZA
NEW BRUNSWICK
NJ
08933-7003
US
|
Family ID: |
34577659 |
Appl. No.: |
10/977086 |
Filed: |
October 29, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60516252 |
Oct 31, 2003 |
|
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|
60558424 |
Mar 31, 2004 |
|
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60558728 |
Mar 31, 2004 |
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Current U.S.
Class: |
205/775 ;
205/777.5 |
Current CPC
Class: |
A61B 5/150358 20130101;
A61B 5/1486 20130101; A61B 5/150022 20130101; A61B 5/150282
20130101; C12Q 1/001 20130101; C12Q 1/006 20130101; Y02A 90/10
20180101; A61B 5/150503 20130101; A61B 5/14532 20130101; G01N
27/3274 20130101; A61B 5/150435 20130101 |
Class at
Publication: |
205/775 ;
205/777.5 |
International
Class: |
G01N 027/26 |
Claims
What is claimed is:
1. A method of reducing interferences in an electrochemical sensor
comprising: measuring a first current at a first working electrode,
said first working electrode being covered by an active reagent
layer; measuring a second current at a second working electrode,
said second working electrode being covered by an inactive reagent
layer; and calculating a corrected current value representative of
a glucose concentration which subtracts said second current from
said first current.
2. A method of reducing interferences in an electrochemical sensor
comprising: measuring a first current at a first working electrode,
said first working electrode being covered by an active reagent
layer; measuring a second current at a second working electrode,
wherein said active reagent layer is disposed on an active region
of said second working electrode and an inactive region of said
second working electrode is covered by an inactive reagent layer;
and calculating a corrected current value representative of a
glucose concentration using a ratio of an active region on said
first and second working electrodes and an inactive region on said
second working electrode.
Description
PRIORITY
[0001] The present invention claims priority to the following US
Provisional Applications: U.S. Provisional Application Ser. No.
60/516,252 filed on Oct. 31, 2003; U.S. Provisional Application
Ser. No. 60/558,424 filed on Mar. 31, 2004; and U.S. Provisional
Application Ser. No. 60/558,728 filed on Mar. 31, 2004. Which
applications are hereby incorporated herein by reference.
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-5027 USNP], filed on Oct. 29, 2004; U.S. patent
application Ser. No. ______ [Attorney Docket Number DDI-5042 USNP],
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; and U.S. patent application Ser.
No. ______ [Attorney Docket Number DDI-5066], filed on Oct. 29,
2004.
FIELD OF THE INVENTION
[0003] The present invention is related, in general to methods of
reducing the effect of interfering compounds on measurements taken
by analyte measurement systems and, more particularly, to a method
of reducing the effects of direct interference currents and
mediated interference currents in a glucose monitoring system using
an electrochemical strip having electrodes with regions coated with
active reagent and regions coated with inactive reagent.
BACKGROUND OF INVENTION
[0004] In many cases, an electrochemical glucose measuring system
may have an elevated oxidation current due to the oxidation of
interfering compounds commonly found in physiological fluids such
as, for example, acetaminophen, ascorbic acid, bilirubin, dopamine,
gentisic acid, glutathione, levodopa, methyldopa, tolazimide,
tolbutamide, and uric acid. The accuracy of glucose meters may,
therefore, be improved by reducing or eliminating the portion of
the oxidation current generated by interfering compounds. Ideally,
there should be no oxidation current generated from any of the
interfering compounds so that the entire oxidation current would
depend only on the glucose concentration.
[0005] It is, therefore, desirable to improve the accuracy of
electrochemical sensors in the presence of potentially interfering
compounds such as, for example, ascorbate, urate, and,
acetaminophen, commonly found in physiological fluids. Examples of
analytes for such electrochemical sensors may include glucose,
lactate, and fructosamine. Although glucose will be the main
analyte discussed, it will be obvious to one skilled in the art
that the invention set forth herein may also be used with other
analytes.
[0006] Oxidation current may be generated in several ways. In
particular, desirable oxidation current results from the
interaction of the mediator with the analyte of interest (e.g.,
glucose) while undesirable oxidation current is generally comprised
of interfering compounds being oxidized at the electrode surface
and by interaction with the mediator. For example, some interfering
compounds (e.g., acetominophen) are oxidized at the electrode
surface. Other interfering compounds (e.g., ascorbic acid) are
oxidized by chemical reaction with the mediator. This oxidation of
the interfering compound in a glucose measuring system causes the
measured oxidation current to be dependent on the concentration of
both the glucose and any interfering compound. Therefore, in the
situation where the concentration of interfering compound oxidizes
as efficiently as glucose and the interferent concentration is high
relative to the glucose concentration, the measurement of the
glucose concentration would be improved by reducing or eliminating
the contribution of the interfering compounds to the total
oxidation current.
[0007] One known strategy that can be used to decrease the effects
of interfering compounds is to use a negatively charged membrane to
cover the working electrode. As an example, a sulfonated
fluoropolymer such as NAFION.TM. may be used to repel all
negatively charged chemicals. In general, most interfering
compounds such as ascorbate and urate have a negative charge, thus,
the negatively charged membrane prevents the negatively charged
interfering compounds from reaching the electrode surface and being
oxidized at that surface. However, this technique is not always
successful since some interfering compounds such as acetaminophen
do not have a net negative charge, and thus, can pass through a
negatively charged membrane. Nor would this technique reduce the
oxidation current resulting from the interaction of interfering
compounds with some mediators. The use of a negatively charged
membrane on the working electrode could also prevent some commonly
used mediators, such as ferricyanide, from passing through the
negatively charged membrane to exchange electrons with the
electrode.
[0008] Another known strategy that can be used to decrease the
effects of interfering compounds is to use a size selective
membrane on top of the working electrode. As an example, a 100
Dalton exclusion membrane such as cellulose acetate may be used to
cover the working electrode to exclude all chemicals with a
molecular weight greater than 100 Daltons. In general, most
interfering compounds have a molecular weight greater than 100
Daltons, and thus, are excluded from being oxidized at the
electrode surface. However, such selective membranes typically make
the test strip more complicated to manufacture and increase the
test time because the oxidized glucose must diffuse through the
selective membrane to get to the electrode.
[0009] Another strategy that can be used to decrease the effects of
interfering compounds is to use a mediator with a low redox
potential, for example, between about -300 mV and +100 mV (when
measured with respect to a saturated calomel electrode). Because
the mediator has a low redox potential, the voltage applied to the
working electrode may also be relatively low which, in turn,
decreases the rate at which interfering compounds are oxidized by
the working electrode. Examples of mediators having a relatively
low redox potential include osmium bipyridyl complexes, ferrocene
derivatives, and quinone derivatives. A disadvantage of this
strategy is that mediators having a relatively low potential are
often difficult to synthesize, unstable and have a low water
solubility.
[0010] Another known strategy that can be used to decrease the
effects of interfering compounds is to use a dummy electrode which
is coated with a mediator. In some instances the dummy electrode
may also be coated with an inert protein or deactivated redox
enzyme. The purpose of the dummy electrode is to oxidize the
interfering compound at the electrode surface and/or to oxidize the
mediator reduced by the interfering compound. In this strategy, the
current measured at the dummy electrode is subtracted from the
total oxidizing current measured at the working electrode to remove
the interference effect. A disadvantage of this strategy is that it
requires that the test strip include an additional electrode and
electrical connection (i.e., the dummy electrode) which cannot be
used to measure glucose. The inclusion of dummy electrode is an
inefficient use of an electrode in a glucose measuring system.
SUMMARY OF INVENTION
[0011] The present invention is directed to a method of reducing
interferences in an electrochemical sensor wherein the method
includes the step of measuring a first current at a first working
electrode, the first working electrode being covered by an active
reagent layer, the step of measuring a second current at a second
working electrode, the second working electrode being covered by an
inactive reagent layer and the step of calculating a corrected
current value representative of a glucose concentration using a
ratio of an active area of the first working electrode to an
inactive area of the second working electrode.
[0012] The present invention is further directed to a method of
reducing interferences in an electrochemical sensor wherein the
method includes the step of measuring a first current at a first
working electrode, the first working electrode being covered by an
active reagent layer, the step of measuring a second current at a
second working electrode, wherein the active reagent layer is
disposed on an active region of the second working electrode and an
inactive region of the second working electrode is covered by an
inactive reagent layer and the step of calculating a corrected
current value representative of a glucose concentration using a
ratio of an active region on the first and second working
electrodes and an inactive region on the second working
electrode.
BRIEF DESCRIPTION OF DRAWINGS
[0013] 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:
[0014] FIG. 1 is an exploded perspective view of a test strip
according to an exemplary embodiment of the present invention;
[0015] FIG. 2 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 1 including a conductive layer and an
insulation layer;
[0016] FIG. 3 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 1, wherein the position of an active and an
inactive reagent layer do not touch each other and are illustrated
with the insulation and conductive layer;
[0017] FIG. 4 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 1, wherein the position of the active and the
inactive reagent layer are immediately adjacent to each other and
are illustrated with the insulation and conductive layer;
[0018] FIG. 5 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 1, wherein the position of the active and the
inactive reagent layer that overlap with each other and are
illustrated with the insulation and conductive layer;
[0019] FIG. 6 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 1, wherein the active and the inactive reagent
layer do not touch each other and are illustrated with the
conductive layer;
[0020] FIG. 7 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 1, wherein the active and the inactive reagent
layer are immediately adjacent to each other and are illustrated
with the conductive layer;
[0021] FIG. 8 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 1, wherein the active and the inactive reagent
layer overlap with each other and are illustrated with the
conductive layer;
[0022] FIG. 9 is a simplified schematic showing a meter interfacing
with a test strip that has a first contact, second contact, and
reference contact disposed on a substrate;
[0023] FIG. 10 is a simplified schematic showing a meter
interfacing with a test strip that has a first contact and a second
contact disposed on a substrate and a reference contact which is
orientated in a facing orientation with the first contact and
second contact;
[0024] FIG. 11 is a graph showing the effects of gamma radiation on
precision for test strips tested at a 20 mg/dL glucose
concentration;
[0025] FIG. 12 is a graph showing the effects of gamma radiation on
precision for test strips tested at a 50 mg/dL glucose
concentration;
[0026] FIG. 13 is a graph showing the effects of gamma radiation on
precision for test strips tested at a 100 mg/dL glucose
concentration;
[0027] FIG. 14 is a graph showing the effects of gamma radiation on
precision for test strips tested at a 300 mg/dL glucose
concentration;
[0028] FIG. 15 is a graph showing the effects of gamma radiation on
precision for test strips tested at a 500 mg/dL glucose
concentration;
[0029] FIG. 16 is a graph showing the effects of gentisic acid on
accuracy for test strips tested at a 70 mg/dL glucose
concentration;
[0030] FIG. 17 is a graph showing the effects of gentisic acid on
accuracy for test strips tested at a 240 mg/dL glucose
concentration;
[0031] FIG. 18 is a graph showing the effects of uric acid on
accuracy for test strips tested at a 70 mg/dL glucose
concentration;
[0032] FIG. 19 is a graph showing the effects of uric acid on
accuracy for test strips tested at a 240 mg/dL glucose
concentration;
[0033] FIG. 20 a simplified plane view of a distal portion of a
test showing a modified cutout that allows the area of a second
working electrode to be increased;
[0034] FIG. 21 is an exploded perspective view of a test strip
according to another exemplary embodiment of the present
invention;
[0035] FIG. 22 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 21, wherein the position of an active and an
inactive reagent layer do not touch each other and are illustrated
with the insulation and conductive layer;
[0036] FIG. 23 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 21, wherein the position of the active and the
inactive reagent layer are immediately adjacent to each other and
are illustrated with the insulation and conductive layer;
[0037] FIG. 24 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 21, wherein the position of the active and the
inactive reagent layer that overlap with each other and are
illustrated with the insulation and conductive layer;
[0038] FIG. 25 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 21, wherein the active and the inactive reagent
layer do not touch each other and are illustrated with the
conductive layer;
[0039] FIG. 26 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 21, wherein the active and the inactive reagent
layer are immediately adjacent to each other and are illustrated
with the conductive layer; and
[0040] FIG. 27 is a simplified plane view of a distal portion of a
test strip according to the embodiment of the present invention
illustrated in FIG. 21, wherein the active and the inactive reagent
layer overlap with each other and are illustrated with the
conductive layer;
DETAILED DESCRIPTION OF THE INVENTION
[0041] The invention described herein includes a test strip to
improve the accuracy of a glucose measurement in the presence of
interfering compounds. Under certain circumstances, a type of
interfering compound may develop in the test strip itself before
bodily fluid such as, for example, blood is added. An example this
type of interfering compound may be a reduced mediator (e.g.
ferrocyanide) which develops from the conversion of an oxidized
mediator (e.g. ferricyanide). This causes the background signal to
increase which, in turn, decreases the accuracy of the test strip
measurement. It should be noted that in this circumstance the
interfering compound develops in the test strip itself as opposed
to being provided to the test strip in the form of a bodily
fluid.
[0042] Typically, an oxidized mediator is disposed on a working
electrode with the intent that the oxidized mediator will be stable
and not transition over to the reduced redox state. The generation
of reduced mediator causes the background signal to increase for
electrochemical sensors which use an oxidation current to correlate
with the glucose concentration. In general, ferricyanide (e.g.
oxidized mediator) tends to become reduced over time to the reduced
redox state. Ferricyanide generally transitions to the reduced
redox state more rapidly when exposed to environmental conditions
which include but are not limited to, basic pH, elevated
temperature, elevated humidity, bright light conditions, electron
beam radiation, and gamma radiation.
[0043] Recently, a lance and a test strip have been integrated into
a single medical device. These integrated medical devices can be
employed, along with an associated meter, to monitor various
analytes, including glucose. Depending on the situation, test
strips can be designed to monitor analytes in an episodic
single-use format, semi-continuous format, or continuous format.
The integration of the lance and the test strip simplifies a
monitoring procedure by eliminating the need for a user to
coordinate the extraction of a bodily fluid from a sample site with
the subsequent transfer of that bodily fluid to the test strip. In
such a case, the lance and test strip must be sterilized together
so as to mitigate the risk of infection.
[0044] Ionizing radiation may be used to sterilize test strips with
a lance. Possible sources of ionizing radiation are electron beam,
gamma, and x-ray. However, one of the challenges in sterilizing a
test strip is to provide a sufficiently high intensity of radiation
such that a sufficiently high proportion of microorganisms are
neutralized for an entire package of test strips, while at the same
time not adversely affecting the reagent layer. Typically, a batch
or package of test strips are exposed to an ionizing radiation dose
ranging from about 10 KGy to about 50 KGy. For the case using
e-beam sterilization, the energy of the incident e-beam source can
range from about 3 MeV to about 12 MeV. The impingent ionizing
radiation may often have some non-uniformities in its intensity
causing a particular portion of the package to receive more
ionizing radiation than another portion of the package. Experiments
have shown that both gamma radiation and electron beam radiation
cause the background signal of the electrochemical sensors to
increase. Furthermore, the relatively non-uniform nature of the
radiation causes the background signal to increase in a non-uniform
nature for a sterilized batch of test strips. This causes the
precision to decrease when testing a particular batch of sterilized
glucose test strips. In addition, the decrease in precision is
exacerbated at the low glucose concentration range (e.g. about 20
mg/dL to about 100 mg/dL) because the proportion of reduced
mediator is relatively high with respect to the low glucose
concentration range.
[0045] FIG. 1 shows an exploded perspective view of a test strip
800 that is designed to compensate for the variations in increased
background potentially caused by the conversion of oxidized
mediator to reduced mediator. In the embodiment of the present
invention illustrated in FIG. 1, an electrochemical test strip 800,
which may be used for measuring glucose concentration in bodily
fluids such as blood or interstitial fluid, includes a first
working electrode 808, a second working electrode 806, and a
reference electrode 810. An active reagent layer 820 is disposed on
first working electrode 808 and reference electrode 810 where
active reagent layer 820 completely covers first working electrode
808 and at least partially covers reference electrode 810. An
inactive reagent layer 818 is disposed on second working electrode
806.
[0046] In an embodiment of this invention, active reagent layer 820
may include, for example, glucose oxidase and a mediator such as,
for example, ferricyanide. Inactive reagent layer 818 may include a
mediator, but no active enzymes which are specific for the analyte
of interest. Because ferricyanide has a redox potential of
approximately 400 mV (when measured with respect to a saturated
calomel electrode) at a carbon electrode, the introduction of a
bodily fluid e.g., blood may generate a significant and undesirable
oxidation of interferents by the mediator and/or the working
electrode. Therefore, the oxidation current measured at first
working electrode 808 will be a superposition of oxidation current
sources: a first, desirable, oxidation current generated by the
oxidation of glucose; a second, undesirable, direct oxidation of
interferents at the electrode (direct interference current); and a
third, undesirable, indirect oxidation of interferents via a
mediator (mediated interference current). The oxidation current
measured at second working electrode 806 will also be a
superposition of oxidation current sources similar to first working
electrode 808, but the first, desirable, oxidation current should
not occur because there is no enzyme present on second working
electrode 806. Because the oxidation current measured at second
working electrode 806 depends only on interferents, and the
oxidation current measured at first working electrode 808 depends
on glucose and interferents, it is possible to calculate a
corrected glucose current which is independent to the effects of
interfering compounds oxidized at first working electrode 808 and
second working electrode 806. In such a case, the current density
of first working electrode 808 is subtracted from the current
density of the second working electrode 806 to calculate a
corrected glucose current density G where
G=WE.sub.1-WE.sub.2 (Eq 8)
[0047] where WE.sub.1 is the current density at first working
electrode 808 and WE.sub.2 is the current density at second working
electrode 806.
[0048] In an alternative embodiment to this invention, the
interferent oxidation current density at second working electrode
806 may be slightly different than the interferent oxidation
current density at first working electrode 808 because there is no
enzyme on second working electrode 806. In such a case, a constant
K can be used to correct for such non-idealities in the correction
method. Equation 9 shows how constant K would modify the previously
described Equation 8.
G=WE.sub.1-(K.times.WE.sub.2) (Eq 9)
[0049] where K can range from about 0.5 to about 1.5.
[0050] Test strip 800 includes a substrate 50, a conductive layer
802, an insulation layer 804, inactive reagent layer 818, active
reagent layer 820, an adhesive layer 830, and a top layer 824. Test
strip 800 may be manufactured by sequentially printing five layers
which are conductive layer 802, insulation layer 804, inactive
reagent layer 818, active reagent layer 820, and adhesive layer 830
onto substrate 50. Top layer 824 may be assembled by a lamination
process. Test strip 800 further includes a first side 54, a second
side 56, a distal portion 58, and a proximal portion 60.
[0051] In one embodiment of the present invention, substrate 50 is
an electrically insulating material such as plastic, glass,
ceramic, and the like. In an embodiment of this invention,
substrate 50 may be a plastic such as, for example, nylon,
polycarbonate, polyimide, polyvinylchloride, polyethylene,
polypropylene, PETG, or polyester. More particularly the polyester
may be, for example Melinex.RTM. ST328 which is manufactured by
DuPont Teijin Films. Substrate 50 may also include an acrylic
coating which is applied to one or both sides to improve ink
adhesion.
[0052] The first layer deposited on substrate 50 is conductive
layer 802 which includes first working electrode 808, second
working electrode 806, reference electrode 810, and strip detection
bar 17. In accordance with the present invention, a screen mesh
with an emulsion pattern may be used to deposit a material such as,
for example, a conductive carbon ink in a defined geometry as
illustrated in FIG. 1. Conductive layer 802 may be disposed on
substrate 50 by using screen printing, rotogravure printing,
sputtering, evaporation, electroless plating, ink jetting,
sublimation, chemical vapor deposition, and the like. Suitable
materials which may be used for conductive layer 802 are Au, Pd,
Ir, Pt, Rh, stainless steel, doped tin oxide, carbon, and the like.
In an embodiment of this invention, the carbon ink layer may have a
height between 1 and 100 microns, more particularly between 5 and
25 microns, and yet even more particularly at approximately 13
microns. The height of conductive layer 802 can vary depending on
the desired resistance and conductivity of the printed
material.
[0053] A first contact 814, a second contact 812, and a reference
contact 816 may be used to electrically interface with a meter.
This allows the meter to electrically communicate to first working
electrode 808, second working electrode 806, and reference
electrode 810 via, respective, first contact 814, second contact
812, and reference contact 816.
[0054] The second layer deposited on substrate 50 is insulation
layer 804. Insulation layer 804 is disposed on at least a portion
of conductive layer 802 as shown in FIGS. 1 and 2. FIG. 2 is a
simplified plane view of distal portion 58 of test strip 800 which
highlights the position of first working electrode 808, second
working electrode 806, and reference electrode 810 with respect to
insulation layer 804. Insulation layer 804 further includes a
cutout 18 which may have a rectangular shaped structure as shown in
FIGS. 1 and 2. Cutout 18 exposes a portions of first working
electrode 808, second working electrode 806, and reference
electrode 810 which can be wetted with liquid. Cutout 18 includes a
cutout width W20 and a cutout length L26. Cutout width W20
corresponds to a width of second working electrode 806, reference
electrode 810, and first working electrode 808 as illustrated in
FIG. 2. In an embodiment of this invention, cutout width W20 may
range from about 0.7 mm to about 1.4 mm, and cutout length L26 may
range from about 0.4 mm and about 3.4 mm.
[0055] In one embodiment of the present invention, second working
electrode 806 and first working electrode 808 have a respective
length of L20 and L21 which may be the same and range from about
0.1 mm to about 0.8 mm. Reference electrode 810 may have a length
L24 which may range from about 0.2 mm to about 1.6 mm. In
accordance with the present invention, electrode spacing S1 is a
distance between second working electrode 806 and reference
electrode 810; and between reference electrode 810 and first
working electrode 808 which may range from about 0.2 mm to about
0.6 mm.
[0056] In an alternative embodiment of the present invention, an
area of first working electrode 808 may be different than an area
of second working electrode 806. A ratio of first working electrode
808 area:second working electrode 806 area may range from about 1:1
to about 1:3. Under certain situations, the reduction in background
can be improved by increasing the relative area of second working
electrode 806. The area of second working electrode 806 may be
increased by modifying the geometry of a cutout 6008 as shown in
FIG. 20.
[0057] FIG. 2 shows that strip 800 may be cut along incision line
A-A' after it is fully laminated as illustrated in FIG. 1. In the
process of cutting test strip 800 along incision line A-A' as
illustrated in FIG. 1, a sample inlet 52 is created in which a
liquid sample can be applied for dosing test strip 800.
[0058] FIGS. 3 to 5 are a simplified plane view of distal portion
58 of test strip 800 according to the embodiment of the present
invention illustrated in FIG. 1, which show various positions of
active reagent layer 820 and inactive reagent layer 818 with
respect to each other. FIGS. 6 to 8, which correspond to FIGS. 3 to
5 respectively, do not show insulation layer 804 to help
demonstrate more clearly the relationship between the conductive
layer 802, active reagent layer 820, and inactive reagent layer
818.
[0059] Test strip 800 may have inactive reagent layer 818 disposed
on second working electrode 806 such that it completely covers
second working electrode 806 as is illustrated in FIGS. 3 to 5. In
one embodiment of this invention, inactive reagent layer 818
completely covers second working electrode 806, but does not touch
reference electrode 810 as is illustrated in FIGS. 3 and 4. In
another embodiment of this invention, inactive reagent layer 818
completely covers second working electrode 806 and at least
partially covers reference electrode 810 as is illustrated in FIG.
5.
[0060] In an embodiment of this invention, inactive reagent layer
818 includes at least an oxidized mediator, such as ferricyanide,
and may optionally include an inert protein or inactivated enzyme.
Inactive reagent layer 818 may further include a citrate buffer at
pH 6, a polyvinyl alcohol, a polyvinyl pyrrolidone-vinyl acetate, a
Dow Corning DC1500 antifoam, a hydroxyethyl cellulose (Natrosol
250G, Hercules), and a surface modified silica (Cab-o-sil TS 610,
Cabot) having both hydrophilic and hydrophobic domains. Examples of
oxidized mediators may be ferricyanide, ferricinium complexes,
quinone complexes, and osmium complexes. Examples of inert protein
may be crotein or albumin (e.g. bovine or human). Examples of
inactivated enzyme may be the apo form of PQQ-glucose dehydrogenase
(where PQQ is an acronym for pyrrolo-quinoline-quinone) or apo
glucose oxidase (e.g. enzyme with no active site). Enzyme may also
be deactivated or sufficiently attenuated by heat treatment or by
treatment with denaturing agents such as urea. Because inactive
reagent layer 818 does not include an active enzyme, the oxidation
current measured at second working electrode 806 is not
proportional to the glucose concentration. For this reason, one
skilled in the art may refer to second working electrode 806 as a
dummy electrode.
[0061] In an embodiment of this invention, the inert protein or
deactivated enzyme in inactive reagent layer 818 may act as a
stabilizer for the mediator. The inert protein or deactivated
enzyme may shield the mediator during the drying process at
elevated temperature. In addition, the inert protein or deactivated
enzyme may act as a desiccant which helps protect the mediator from
moisture that may potentially destabilize the mediator.
[0062] Test strip 800 has active reagent layer 820 disposed on
first working electrode 808 as illustrated in FIGS. 3 to 5. In
another embodiment of this invention, active reagent layer 820
completely covers first working electrode 808, but does not touch
reference electrode 810. In another embodiment of this invention,
active reagent layer 820 completely covers first working electrode
808 and at least partially covers reference electrode 810 as
illustrated in FIGS. 3 to 5.
[0063] In an embodiment of this invention, active reagent layer 820
includes at least an oxidized mediator, and an enzyme. Active
reagent layer 820 may further include a citrate buffer at pH 6, a
polyvinyl alcohol, a polyvinyl pyrrolidone-vinyl acetate, a Dow
Corning DC1500 antifoam, a hydroxyethyl cellulose (Natrosol 250G,
Hercules), and a surface modified silica (Cab-o-sil TS 610, Cabot)
having both hydrophilic and hydrophobic domains. Examples of
oxidized mediators may be ferricyanide, ferricinium complexes,
quinone complexes, and osmium complexes. Examples of the enzyme may
be glucose oxidase, glucose dehydrogenase using a PQQ co-factor,
and glucose dehydrogenase using a nicotinamide adenine dinucleotide
co-factor. Because active reagent layer 820 does include the
enzyme, the oxidation current measured at first working electrode
808 is proportional to the glucose concentration.
[0064] It should be noted that if screen printing were used for
depositing both inactive reagent layer 818 and active reagent layer
820, then two separate screen printing steps would be required to
deposit the respective reagent layers onto the appropriate
electrode(s). It should be noted that screen printing is not
well-suited for printing two discrete reagents on the same screen.
The squeegee motion during printing may cause the two respective
reagents to mix during the screen printing process. FIG. 3 shows an
embodiment of this invention which has inactive reagent layer 818
disposed on second working electrode 806, and active reagent layer
820 disposed on first working electrode 808 and reference electrode
810. In this embodiment, inactive reagent layer 818 does not touch
or overlap with active reagent layer 820. Because the area of
second working electrode 806, first working electrode 808 and
reference electrode 810 is relatively small, it can be difficult to
sequentially align and coat inactive reagent layer 818 and active
reagent layer 820, respectively, with the desired yield. It should
also be noted that relatively small electrode areas (e.g. about 0.6
mm.sup.2) are preferred because this allows the volume of liquid
sample required for a test strip to be small.
[0065] In an embodiment of this invention, inactive reagent layer
818 is printed first and then dried at an elevated temperature.
Active reagent layer 820 is then subsequently printed followed by
another drying step at an elevated temperature as described in
International Application serial number PCT/GB/03004708 which is
hereby incorporated by reference herein. Because active reagent
layer 820 is deposited second, it is exposed to only one drying
step as opposed to the two drying steps for inactive reagent layer
818. This helps stabilize both mediator and enzyme within active
reagent layer 820 because under certain conditions enzymes can
degrade with continued exposure to elevated temperatures.
[0066] In an embodiment of this invention, FIG. 4 shows inactive
reagent layer 818 disposed on second working electrode 806, and
active reagent layer 820 disposed on first working electrode 808
and reference electrode 810. In this embodiment inactive reagent
layer 818 and active reagent layer 820 are immediately adjacent to
each other. In such a case, the inactive reagent layer 818 and
active reagent layer 820 would touch, but typically not overlap
with each other to any significant extent. Although the printing
process targets the alignment such that inactive reagent layer 818
and active reagent layer 820 are immediately adjacent to each
other, normal manufacturing variation will cause some overlap to
occur with a certain frequency between inactive reagent layer 818
and active reagent layer 820. Likewise, such variation will also
cause inactive reagent layer 818 to sometimes not touch active
reagent layer 820. Because inactive reagent layer 818 was allowed
to touch or not touch active reagent layer 820 and the operation of
the method of the invention still works to reduce the variation in
the background in either circumstance, the yield of acceptable test
strips was improved.
[0067] It should be noted that the overlap of inactive reagent
layer 818 with active reagent layer 820 does not affect the glucose
measurement as long as the enzyme from active reagent layer 820
cannot diffuse, to any significant extent in the time allowed for
the measurement (i.e. about 5 seconds or less), to second working
electrode 806. If enzyme were to diffuse to second working
electrode 806, then first working electrode 808 would measure a
glucose current in addition to the non-enzyme specific currents.
This would prevent test strip 800 from effectively reducing the
background signal.
[0068] It should also be noted that if the overlap of inactive
reagent layer 818 with active reagent layer 820 were to occur on
reference electrode 810 that this would not affect the glucose
measurement. In such a case, the amount of enzyme and/or oxidized
mediator on reference electrode 810 will increase, but should not
affect the glucose measurement or the background correction
algorithm.
[0069] Yet another embodiment of this invention which improves upon
the method of coating inactive reagent layer 818 and active reagent
layer 820 is shown in FIG. 5 Inactive reagent layer 818 may be
coated such that it completely covers second working electrode 806
and a portion of reference electrode 810. Similarly, active reagent
layer 820 may be coated such that it completely covers first
working electrode 808 and at least a portion of reference electrode
810. In an embodiment of this invention, the printing process can
target the alignment such that inactive reagent layer 818 and
active reagent layer 820 substantially overlap with each other on
reference electrode 810 at an overlap zone 822. In such a case,
inactive reagent layer 818 and active reagent layer 820 may mix
with each other at overlap zone 822. Because the length of both
inactive reagent layer 818 and active reagent layer 820 was further
increased compared to the embodiment described in FIG. 4, the
alignment and coating of active reagent layer 820 and inactive
reagent layer 818 to first working electrode 808 and second working
electrode 806 was yet further improved.
[0070] It should be noted that second working electrode 806 (e.g.
dummy electrode) is located on distal portion 58 of test strip 800
as illustrated in FIGS. 1 to 5. This causes the physiological fluid
to sequentially wet in the following order--second working
electrode 806, reference electrode 810, and then first working
electrode 808. Test strip 800 was purposefully designed to have
inactive reagent layer 818 (which contains no enzyme) upstream of
active reagent layer 820 (which does contain enzyme). This reduces
the possibility of enzyme being present at both second working
electrode 806 and first working electrode 808. If active reagent
layer 820, which contains enzyme, was coated over second working
electrode 806, and no enzyme were present over first working
electrode 808 then it would be possible that some enzyme could be
swept to first working electrode 808 from second working electrode
806. The presence of a significant amount of enzyme on first
working electrode 808 would prevent the background signal from
being reduced through the use of the dummy electrode format.
[0071] In an embodiment of this invention, top layer 824 may be in
the form of an integrated lance 826 as shown in FIG. 1. In such an
embodiment, top layer 824 may include a lance 826 which is located
at distal portion 58. Lance 826, which may also be referred to as a
penetration member, may be adapted to pierce a user's skin and draw
blood into test strip 800 such that second working electrode 806,
first working electrode 808, and reference electrode 810 are
wetted. Top layer 824 is adhered to test strip 800 by adhesive
layer 830. This adhesive layer 830 can be a heat seal or a pressure
sensitive adhesive. Lance 826 includes a lancet base 832 that
terminates at distal portion 58 of assembled test strip 800. Lance
826 may be made with either an insulating material such as plastic,
glass, and silicon, or a conducting material such as stainless
steel and gold. For the case in which top layer 824 is conductive,
top layer 824 may also be used as a reference electrode 810 which
is orientated with a facing relationship to second working
electrode 806 and first working electrode 808. Further descriptions
of integrated medical devices that use an integrated lance can be
found in International Application No. PCT/GB01/05634 and U.S.
patent application Ser. No. 10/143,399 which are hereby fully
incorporated by reference herein. In addition, lance 826 can be
fabricated, for example, by a progressive die-stamping technique,
as disclosed in the aforementioned International Application No.
PCT/GB01/05634 and U.S. patent application Ser. No. 10/143,399
which are hereby fully incorporated by reference herein.
[0072] In an embodiment of the present invention, adhesive layer
830 has a height of about 70 to 110 microns. Adhesive layer 830 may
include a double sided pressure sensitive adhesive, a UV cured
adhesive, heat activated adhesive, thermosetting plastic, or other
adhesive known to those skilled in the art. As a non-limiting
example, adhesive layer 830 may be formed by screen printing a
pressure sensitive adhesive such as, for example, a water based
acrylic copolymer pressure sensitive adhesive which is commercially
available from Tape Specialties LTD in Tring, Herts, United Kingdom
(part#A6435).
[0073] In a method of this invention, the background variations are
reduced by subtracting a first current from first working electrode
808 from a second current from second working electrode 806. To
initiate a test, a sample is applied to sample inlet 52 which
allows a current to be measured at second working electrode 806 and
first working electrode 808. Because second working electrode 806
does not have a glucose oxidizing enzyme disposed thereon, a
magnitude of an oxidation current at second working electrode 806
is proportional to an amount of interfering compounds present on
test strip 800 and also an amount of interfering compounds
originating from the sample. This allows a corrected current value
to be calculated using a difference between first working electrode
808 and second working electrode 806 to reduce the effects of
interfering compounds present in the sample and also for
interfering compounds that may be present on test strip 800.
[0074] FIG. 9 is a simplified schematic showing a meter 900
interfacing with test strip 800. Meter 900 has at least three
electrical contacts that form an electrical connection to second
working electrode 806, first working electrode 808, and reference
electrode 810. In particular second contact 812 and reference
contact 816 connect to first voltage source 910; first contact 814
and reference contact 816 connect to second voltage source 920.
When performing a test, first voltage source 910 applies a first
potential E1 between second working electrode 806 and reference
electrode 810; and second voltage source 920 applies a second
potential E2 between first working electrode 808 and reference
electrode 810.
[0075] In one embodiment of this invention, first potential E1 and
second potential E2 may be the same such as for example about +0.4
V. In another embodiment of this invention, first potential E1 and
second potential E2 may be different. A sample of blood is applied
such that second working electrode 806, first working electrode
808, and reference electrode 810 are covered with blood. This
allows second working electrode 806 and first working electrode 808
to measure a current which is proportional to glucose and/or
non-enzyme specific sources. After about 5 seconds from the sample
application, meter 900 measures an oxidation current for both
second working electrode 806 and first working electrode 808.
[0076] FIG. 10 is a simplified schematic showing a meter 900
interfacing with test strip 800. In contrast to FIG. 9, top layer
824 is conductive and used as a reference electrode instead of
reference electrode 810 which is disposed on substrate 50. More
particularly, FIG. 10 shows that top layer 824, in the form of a
reference electrode, has a facing relationship with first working
electrode 808 and second working electrode 806. In this case, meter
900 forms an electrical contact to top layer 824 instead of at
reference contact 816 as is shown in FIG. 1.
[0077] FIG. 21 is an exploded perspective view of a test strip 1000
according to another embodiment of the present invention. The
oxidation current measured at a first working electrode 100 will be
a superposition of oxidation current sources: a first, desirable,
oxidation current generated by the oxidation of glucose and a
second, undesirable, oxidation current generated by the
interferents. The oxidation of interferents may occur directly at
first working electrode 100 and indirectly through a mediated
mechanism via a mediator.
[0078] Second working electrode 102 has a geometric trace that has
an active portion 102a which is coated with active reagent 820 and
an inactive portion 102i which is coated with inactive reagent 818
as illustrated in FIGS. 22 to 27. The oxidation current sources
measured at active portion 102a will be similar to first working
electrode 100. Inactive portion 102i of second working electrode
102 will oxidize interferents and not oxidize glucose because there
is no enzyme present. Further, inactive portion 102i will oxidize
interferents directly at second working electrode 102 and
indirectly through a mediated mechanism via a mediator. Because the
oxidation current measured at inactive portion 102i does not depend
on glucose and the area of inactive portion 102i is known, it is
possible to calculate its contribution to the interferent oxidation
current measured at second working electrode 102. In turn, using
the interferent oxidation current calculated for inactive portion
102i and knowing the area of first working electrode 100 and the
area of active portion 102a, it is possible to calculate a
corrected glucose current which accounts for the effects of
interfering compounds oxidized at the electrode. It should be noted
that in the present invention, inactive portion 102i helps correct
the glucose current for direct and mediated interference oxidation.
It should also be noted that inactive portion 102i and active
portion 102a may sometimes by referred to as an inactive region and
an active region, respectively.
[0079] An algorithm may, therefore be used to calculate a corrected
glucose current that is independent of interferences. After dosing
a sample onto test strip 1000, a constant potential is applied to
first working electrode 100 and second working electrode 102 and a
current is measured for both electrodes. At first working electrode
100 where active reagent layer 820 covers the entire electrode
area, the following equation can be used to describe the components
contributing to the oxidation current,
WE.sub.1=G+I.sub.1a (Eq 1)
[0080] where WE.sub.1 is a current density at the first working
electrode, G is a current density due to glucose which is
independent of interferences, and I.sub.1a is a current density due
to interferences oxidized at first working electrode 100 which is
covered with active reagent 820.
[0081] At second working electrode 102 which is partially covered
with active reagent 820 and inactive reagent 818, the following
equation can be used to describe the components contributing to the
oxidation current,
WE.sub.2=G+I.sub.2a+I.sub.2i (Eq 2)
[0082] where WE.sub.2 is a current density at the second working
electrode, I.sub.2a is a current density due to interferences at
the active portion 102a, and 12i is a current density due to
interferences at inactive portion 102i.
[0083] To reduce the effects of interferences, an equation is
formulated which describes the relationship between the interferent
current at active portion 102a and inactive portion 102i. It is
approximated that the interferent oxidation current density
measured at active portion 102a is the same as the current density
measured at the inactive portion 102i. This relationship is further
described by the following equation, 1 I 2 a = A 2 a A 2 i .times.
I 2 i ( Eq 3 a )
[0084] where A.sub.2a is an area of second working electrode
covered with active reagent layer 820 and A.sub.2i is an area of
second working electrode covered with inactive reagent layer
818.
[0085] Inactive portion 102i can oxidize interferents, but not
glucose because it is not coated with enzyme. Active portion 102a
can oxidize glucose and interferents. Because it was experimentally
found that inactive portion 102i oxidizes interferents in a manner
proportional to the area of active portion 102a, it is possible to
predict the proportion of interferent current measured overall at
second working electrode 102. This allows the overall current
measured at second working electrode 102 (i.e. WE.sub.2) to be
corrected by subtracting the contribution of the interferent
current. In an embodiment of the present invention the ratio of
A.sub.2i:A.sub.2a may be between about 0.5:1 to 5:1, and is
preferably about 3:1. More details describing this mathematical
algorithm for current correction will be described in the
subsequent sections.
[0086] In an alternative embodiment of the present invention,
I.sub.2a may be different than I.sub.2i. This may be ascribed to a
more efficient or less efficient oxidation of interferents at the
active portion 102a because of the presence of enzyme. For not well
described reasons, it is possible that the presence of enzyme may
affect the electrode's ability to oxidize mediator and/or
interferents. This behavior may be phenomenologically modeled by
re-writing Equation 3a to the following form,
I.sub.2a=f.times.I.sub.2i (Eq 3b)
[0087] where f is a correction factor which incorporates the
effects of the interferent oxidation efficiency of the active
portion 102a to inactive portion 120i.
[0088] In an embodiment of the present invention, Equation 1, 2,
and 3a may be manipulated to derive an equation that outputs a
corrected glucose current density independent of interferences. It
should be noted that the three equations (Equation 1, 2, and 3a)
collectively have 4 unknowns which are G, I.sub.2i, I.sub.2a, and
I.sub.1a. However, I.sub.1a and I.sub.2a can be conservatively
assumed to be equal because they are measured at the same
conductive material and coated with the same active reagent layer
820. Equation 1 can be rearranged to the following form.
G=WE.sub.1-I.sub.1a=WE.sub.1-I.sub.2a (Eq 4)
[0089] Next, I.sub.2a from Equation 3a can be substituted into
Equation 4 to yield Equation 5. 2 G = WE 1 - [ A 2 a A 2 i .times.
I 2 i ] ( Eq 5 )
[0090] Next, Equation 1 and Equation 2 can be combined to yield
Equation 6.
I.sub.2i=WE.sub.2-WE.sub.1 (Eq 6)
[0091] Next, I.sub.2i from Equation 6 can be substituted into
Equation 5 to yield Equation 7a. 3 G = WE 1 - { ( A 2 a A 2 i ) X (
WE 2 - WE 1 ) } ( Eq 7 a )
[0092] Equation 7a outputs a corrected glucose current density G
which removes the effects of interferences requiring only the
measured current density from first working electrode 100 and
second working electrode 102 (i.e. WE.sub.1 and WE.sub.2), and a
proportion of an area of the second working electrode covered with
active reagent to an area of the second working electrode covered
with inactive reagent 4 A 2 a A 2 i
[0093] ). In one embodiment of the present invention the proportion
5 A 2 a A 2 i
[0094] may be programmed into a glucose meter, in, for example, a
read only memory. In another embodiment of the present invention,
the proportion 6 A 2 a A 2 i
[0095] may be transferred to the meter via a calibration code chip
which would may account for manufacturing variations in A.sub.2a or
A.sub.2i.
[0096] In an alternative embodiment to the present invention
Equation 1, 2, and 3b may be used when the interferent oxidation
current density for active portion 102a is different from the
interferent oxidation current density of inactive portion 102i. In
such a case, an alternative correction Equation 7b is derived as
shown below.
G=WE.sub.1-{f.times.(WE.sub.2-WE.sub.1)} (Eq 7b)
[0097] In another embodiment of the present invention, the
corrected glucose current Equation 7a or 7b may be used by the
meter only when a certain threshold is exceeded. For example, if
WE.sub.2 is about 10% or greater than WE.sub.1, then the meter
would use Equation 7a or 7b to correct for the current output.
However, if WE.sub.2 is about 10% or less than WE.sub.1, the meter
would simple take an average current value between WE.sub.1 and
WE.sub.2 to improve the accuracy and precision of the measurement.
The strategy of using Equation 7a or 7b 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 WE.sub.2 is sufficiently greater than WE.sub.1 (e.g.
about 20% or more), this is an indicator of having a sufficiently
high concentration of interferents. In such a case, it may be
desirable to output an error message instead of a glucose value
because a very high level of interferents may cause a breakdown in
the accuracy of Equation 7a or 7b.
[0098] FIG. 21 shows an exploded perspective view of a test strip
embodiment that is designed to compensate for variations in
increased background caused by the conversion of oxidized mediator
to reduced mediator. Test strip 1000 includes a substrate 50, a
conductive layer 164, an insulation layer 106, an inactive reagent
layer 818, an active reagent layer 820, an adhesive layer 830, and
a top layer 824. Test strip 1000 further includes a distal end 58
and a proximal end 60. It should be noted that test strip 1000 is a
modification of test strip 800 so that an active reagent coating
820 covers a portion of both a first working electrode 100 and a
second working electrode 102. This allows for two glucose
measurements to be made while at the same time allows for the
correction of interferents which develop within test strip 1000 or
are dosed into test strip 1000. Test strip 1000 would employ either
Equation 7a or 7b for reducing the effect of interfering compounds
or increased background. In contrast to test strip 800, test strip
1000 has a modification to conductive layer 164 and insulation
layer 106. Substrate 50, inactive reagent layer 818, active reagent
layer 820, adhesive layer 830 and top layer 824 are similar in both
shape and material for both test strip 1000 and test strip 800.
[0099] FIGS. 22 to 24 are a simplified plane view of distal portion
58 of test strip 1000, according to the embodiment of the present
invention illustrated in FIG. 21, which show various positions of
active reagent layer 820 and inactive reagent layer 818 with
respect to each other. FIGS. 25 to 27, which correspond to FIGS. 22
to 24 respectively, do not show insulation layer 804 to help
demonstrate more clearly the relationship between the conductive
layer 164, active reagent layer 820, and inactive reagent layer
818.
[0100] In test strip 1000, conductive layer 164 is disposed on
substrate 50. Conductive layer 164 includes a first working
electrode 100, a second working electrode 102, a reference
electrode 104, a first contact 101, a second contact 103, a
reference contact 105, a strip detection bar 17, as shown in FIG.
21. In contrast to test strip 800, second working electrode 806 and
first working electrode 102 has a C-shape.
[0101] FIG. 22 is a simplified plane view of first working
electrode 100, second working electrode 102, and reference
electrode 104, insulation layer 106, inactive reagent layer 818,
and active reagent layer 820. Insulation layer 106 includes a
cutout 108 which defines the area of second working electrode 102
to have an inactive portion 102i and an active portion 102a. In
this embodiment, inactive reagent layer 818 was disposed on
inactive portion 102i and active reagent layer 820 was disposed on
active portion 102a, first working electrode 100, and reference
electrode 104. FIG. 22 shows that inactive reagent layer 818 does
not touch or overlap with active reagent layer 820.
[0102] Test strip 1000 differs from test strip 800 in that both
inactive reagent layer 818 and active reagent layer 820 both coat a
portion of second working electrode 102. This allows two glucose
measurements to be performed while at the same time reduce the
effects of background and/or interferences. One of the challenges
with making test strip 1000 as shown in FIG. 22 is that it can be
difficult to sequentially align and coat the respective inactive
reagent layer 818 and active reagent layer 820 so that they do not
touch each other with the desired yield because the area of first
working electrode 100, second working electrode 102 and reference
electrode 104 is relatively small.
[0103] In an embodiment of this invention, FIG. 23 shows inactive
reagent layer 818 disposed on inactive portion 102i, and active
reagent layer 820 disposed on active portion 102a, first working
electrode 100, and reference electrode 104. In this embodiment
inactive reagent layer 818 and active reagent layer 820 are
immediately adjacent to each other. In such an ideal case the
inactive reagent layer 818 and active reagent layer 820 would
touch, but not substantially overlap with each other. Although the
printing process targets the alignment such that inactive reagent
layer 818 and active reagent layer 820 are immediately adjacent to
each other, normal manufacturing variation will cause some overlap
to occur with a certain frequency between inactive reagent layer
818 and active reagent layer 820. Likewise, such variation will
also cause inactive reagent layer 818 to not touch active reagent
layer 820 at a certain frequency. Because inactive reagent layer
818 was allowed to touch or not touch active reagent layer 820, the
yield of acceptable test strips was improved.
[0104] Yet another embodiment of this invention which improves upon
the method of coating inactive reagent layer 818 and active reagent
layer 820 is shown in FIG. 24. Inactive reagent layer 818 may be
coated such that it completely covers inactive portion 102i and a
portion of reference electrode 104. Similarly, active reagent layer
820 may be coated such that it completely covers active portion
102a, first working electrode 100 and at least a portion of
reference electrode 104. In an embodiment of this invention, the
printing process can target the alignment such that inactive
reagent layer 818 and active reagent layer 820 substantially
overlap with each other on reference electrode 810 at an overlap
zone 822. In such a case, inactive reagent layer 818 and active
reagent layer 820 may mix with each other at overlap zone 822.
Because the length of both inactive reagent layer 818 and active
reagent layer 820 was further increased compared to the embodiment
described in FIG. 23, the alignment and coating of active reagent
layer 820 and inactive reagent layer 818 was yet further improved
in terms of manufacturing yield.
[0105] It is an advantage of this invention in that two reagent
layers are used which helps reduce the effects of increased
background. The ability to sufficiently compensate for varying
levels of reduced mediator such as ferrocyanide in the test strip
itself enables a high level of accuracy and precision to be
achieved. There are several factors that may influence the
conversion of oxidized mediator to the reduced form during the
manufacturing, testing, and storage process. Therefore, this allows
for corrections to be made which account for manufacturing
variations such as reagent layer height (within batch and
batch-to-batch), heat seal adhesive manufacturing conditions, high
temperature drying, packaging, and sterilization conditions.
Because the correction accounts for these variation, a more robust
process can be envisaged in which rigorous process controls are not
needed to monitor and control such manufacturing variations. The
measurement of background currents may also improve the stability
of test strip to withstand adverse storage conditions such as high
temperature and humidity. This may allow simpler cartridges to be
designed for storing test strips which may not need a rigorous seal
to withhold moisture.
EXAMPLE 1
[0106] Test strips 800 were prepared as illustrated in FIGS. 1 to
3a. Test strips 800 were tested in blood which were exposed to
varying levels of sterilizing radiation. To test strips 800, they
were electrically connected to a potentiostat which has the means
to apply a constant potential of +0.4 volts between first working
electrode 808 and reference electrode 810; and second working
electrode 806 and the reference electrode 810. A sample of blood is
applied to sample inlet 52 allowing the blood to wick into the
sample receiving chamber and to wet first working electrode 808,
reference electrode 810, and second working electrode 806. Active
layer 820 becomes hydrated with blood and then generates
ferrocyanide which may be proportional to the amount of glucose
and/or interferent concentration present in the sample. In
contrast, inactive layer 818 becomes hydrated with blood and does
not generate additional ferrocyanide that was not present within
inactive layer 818 before hydration. After about 5 seconds from the
sample application to test strip 800, an oxidation of ferrocyanide
and/or interferences are measured as a current for both first
working electrode 808 and second working electrode 806.
EXAMPLE 2
[0107] Two batches of test strips were prepared to show that the
use of inactive reagent layer 818 and active reagent layer 820
improved the overall precision for test strips sterilized by gamma
radiation. Both batches of test strips were tested in a similar
manner as described in Example 1. The first test strip batch is
test strip 800 and is referred to as Batch 1. The second test strip
batch, which is referred to as Batch 2, is also similar to test
strip 800, but does not include inactive reagent layer 818 and also
has a modified active reagent layer which covers both first working
electrode 808, second working electrode 806, and reference
electrode 810. When testing Batch 1, the difference in current from
first working electrode 808 and second working electrode 806 was
used to calculate a corrected signal current which was then
converted to a glucose concentration. When testing Batch 2, the
current from second working electrode 806 and first working
electrode 808 were summed together to determine a value which was
then used to calculate an uncorrected glucose concentration. Before
testing with blood, both Batch 1 and Batch 2 test strips were
treated with 0 kGy and 25 kGy of gamma radiation. Next, the four
test cases, which are Batch 1--0 kGy, Batch 1--25 kGy, Batch 2--0
kGy, and Batch 2--25 kGy, were evaluated for precision by testing
24 test strips with blood for each test case at 5 glucose
concentrations which was 20, 50, 100, 300, and 500 mg/dL.
[0108] FIGS. 11 to 15 show that Batch 1 test strips did not suffer
from a degradation in precision after being sterilized with 25 kGy
of gamma radiation. For all five glucose concentrations, the
precision was substantially similar or better after sterilization
for Batch 1 test strips. This shows that the use of active reagent
layer 820 and inactive reagent layer 818 helps compensate for
background levels of ferrocyanide produced during the sterilization
process.
[0109] FIGS. 11 to 13 show that Batch 2 test strips did suffer from
a degradation in precision after being sterilized with 25 kGy of
gamma radiation. This control experiment verifies that there is a
degradation in precision when not using the background reduction
method of the present invention. Because Batch 2 test strip did not
have inactive reagent layer 818, the background reduction method
could not be implemented. Batch 2 test strips, did not suffer from
a degradation in precision after being sterilized because
relatively high glucose concentrations were tested (300 and 500
mg/dL) in which the effect of sterilization on precision is not as
significant. In this case, the amount of ferrocyanide generated by
glucose oxidase is significantly higher than ferrocyanide generated
(e.g. by sterilization processes) before hydrating the test
strip.
EXAMPLE 3
[0110] Another batch of test strips, which is referred to as Batch
3, was prepared in a manner similar to test strip 800 except that
second working electrode 806 was not coated with either active
reagent layer 820 or inactive reagent layer 818. In this example,
Batches 1 to 3 were tested to evaluate the overall accuracy in the
presence of interfering compounds such as uric acid and gentisic
acid.
[0111] Batch 1, Batch 2, and Batch 3 test strips were tested in
blood at three concentrations of gentisic acid which were 0, 25,
and 50 mg/dL. For each gentisic acid concentration, two glucose
concentrations were tested which were 70 and 240 mg/dL. FIGS. 16
and 17 show that Batch 1 and Batch 3 test strips had an
insignificant change (<10 mg/dL or 10%) in bias when testing
them at 25 and 50 mg/dL gentisic acid concentration. In contrast,
Batch 2 test strips had a significant change (>10 mg/dL or 10%)
in bias when testing them at a 25 and a 50 mg/dL gentisic acid
concentration. This shows that the use of second working electrode
806 not coated with enzyme allows for an effective correction of
the glucose signal in the presence of high concentrations of
gentisic acid.
[0112] Batch 1, Batch 2, and Batch 3 test strips were tested in
blood at three concentrations of uric acid which were 0, 10, and 20
mg/dL. For each uric acid concentration, two glucose concentrations
were tested which were 70 and 240 mg/dL. FIGS. 18 and 19 show that
Batch 1 and Batch 3 test strips had an insignificant change (<10
mg/dL or 10%) in bias when testing them at 10 and 20 mg/dL uric
acid concentration. In contrast, Batch 2 test strips had a
significant change (>10 mg/dL or 10%) in bias when testing them
at a 10 and a 20 mg/dL uric acid concentration. This shows that the
use of second working electrode 806 not coated with enzyme allows
for an effective correction of the glucose signal in the presence
of high concentrations of uric acid.
[0113] 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 which 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.
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