US20050139469A1 - Electrochemical test strip for reducing the effect of direct and mediated interference current - Google Patents

Abstract

The present invention is directed to an electrochemical sensor or electrochemical strip which includes a substrate, a first working electrode disposed on the substrate, a second working electrode disposed on the substrate, a reference electrode, an active reagent layer disposed on the first working electrode, wherein the active reagent layer completely covers the first working electrode and an inactive reagent layer disposed on the second working electrode, wherein the inactive reagent completely covers the second working electrode. The present invention is also directed to an electrochemical sensor an electrochemical sensor including a substrate, a first working electrode disposed on the substrate, a second working electrode disposed on the substrate, a reference electrode, an active reagent layer disposed on the first working electrode, wherein the active reagent layer completely covers the first working electrode, the second working electrode having an active region and an inactive region, the active reagent layer disposed on a active region of the second working electrode and an inactive reagent layer disposed on the inactive region of the second working electrode.

Description

PRIORITY
• The present invention claims priority to the following US Provisional Applications: U.S. Provisional Application Ser. No. 60/516,252 filed Oct. 31, 2003; U.S. Provisional Application Ser. No. 60/558,424 filed Mar. 31, 2004; and U.S. Provisional Application Ser. No. 60/558,728 filed Mar. 31, 2004, which applications are hereby incorporated herein by reference.
RELATED APPLICATIONS
• The present invention is related to the following co-pending US Applications:
• U.S. patent application Ser. No. ______ [Attorney Docket Number DDI-5027], filed on Oct. 29, 2004; 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; and U.S. patent application Ser. No. ______ [Attorney Docket Number DDI-5067], filed on Oct. 29, 2004.
FIELD OF THE INVENTION
• The present invention is related, in general to electrochemical strips and systems which are designed to reduce the effect of interfering compounds on measurements taken by such analyte measurement systems and, more particularly, to an improved electrochemical strip for reducing the effects of direct interference currents and mediated interference currents in a glucose monitoring system wherein the electrochemical strip has electrodes with regions coated by active reagent and regions coated with inactive reagent.
BACKGROUND OF INVENTION
• 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.
• 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.
• 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.
• 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™ 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.
• 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.
• 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.
• 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
• The present invention is directed to an electrochemical sensor or electrochemical strip which includes a substrate, a first working electrode disposed on the substrate, a second working electrode disposed on the substrate, a reference electrode, an active reagent layer disposed on the first working electrode, wherein the active reagent layer completely covers the first working electrode and an inactive reagent layer disposed on the second working electrode, wherein the inactive reagent completely covers the second working electrode. The present invention further includes an electrochemical sensor wherein the first working electrode, the second working electrode and the reference electrode are positioned in a sample receiving chamber, the sample receiving chamber has a proximal and a distal end, the distal end including a first opening which is adapted to receive bodily fluids and the second working electrode being positioned adjacent the first opening. The present invention further includes an electrochemical sensor wherein the first working electrode and the reference electrode are positioned proximal to the second working electrode.
• The present invention is also directed to an electrochemical sensor an electrochemical sensor including a substrate, a first working electrode disposed on the substrate, a second working electrode disposed on the substrate, a reference electrode, an active reagent layer disposed on the first working electrode, wherein the active reagent layer completely covers the first working electrode, the second working electrode having an active region and an inactive region, the active reagent layer disposed on an active region of the second working electrode and an inactive reagent layer disposed on the inactive region of the second working electrode. The present invention further includes an electrochemical sensor wherein the first working electrode, the second working electrode and the reference electrode are positioned in a sample receiving chamber, the sample receiving chamber having a proximal and a distal end, the distal end including a first opening which is adapted to receive bodily fluids and the inactive region of the second working electrode being positioned adjacent the first opening. The present invention further includes an electrochemical sensor wherein the active region of the second working electrode and the first working electrode are positioned proximal to the inactive region of the second working electrode.
BRIEF DESCRIPTION OF DRAWINGS
• 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:
• FIG. 1 is an exploded perspective view of a test strip according to an exemplary embodiment of the present invention;
• FIG. 2 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 1 including a conductive layer and an insulation layer;
• FIG. 3 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 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;
• FIG. 4 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 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;
• FIG. 5 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 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;
• FIG. 6 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 1, wherein the active and the inactive reagent layer do not touch each other and are illustrated with the conductive layer;
• FIG. 7 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 1, wherein the active and the inactive reagent layer are immediately adjacent to each other and are illustrated with the conductive layer;
• FIG. 8 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 1, wherein the active and the inactive reagent layer overlap with each other and are illustrated with the conductive layer;
• 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;
• 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;
• FIG. 11 is a graph showing the effects of gamma radiation on precision for test strips tested at a 20 mg/dL glucose concentration;
• FIG. 12 is a graph showing the effects of gamma radiation on precision for test strips tested at a 50 mg/dL glucose concentration;
• FIG. 13 is a graph showing the effects of gamma radiation on precision for test strips tested at a 100 mg/dL glucose concentration;
• FIG. 14 is a graph showing the effects of gamma radiation on precision for test strips tested at a 300 mg/dL glucose concentration;
• FIG. 15 is a graph showing the effects of gamma radiation on precision for test strips tested at a 500 mg/dL glucose concentration;
• FIG. 16 is a graph showing the effects of gentisic acid on accuracy for test strips tested at a 70 mg/dL glucose concentration;
• FIG. 17 is a graph showing the effects of gentisic acid on accuracy for test strips tested at a 240 mg/dL glucose concentration;
• FIG. 18 is a graph showing the effects of uric acid on accuracy for test strips tested at a 70 mg/dL glucose concentration;
• FIG. 19 is a graph showing the effects of uric acid on accuracy for test strips tested at a 240 mg/dL glucose concentration;
• 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;
• FIG. 21 is an exploded perspective view of a test strip according to another exemplary embodiment of the present invention;
• FIG. 22 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 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;
• FIG. 23 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 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;
• FIG. 24 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 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;
• FIG. 25 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 21, wherein the active and the inactive reagent layer do not touch each other and are illustrated with the conductive layer;
• FIG. 26 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 21, wherein the active and the inactive reagent layer are immediately adjacent to each other and are illustrated with the conductive layer; and
• FIG. 27 is a simplified plane view of a distal portion of a test strip according to the embodiment of the present invention illustrated inFIG. 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
• 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.
• 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.
• 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.
• 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.
• FIG. 1 shows an exploded perspective view of atest strip800 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 inFIG. 1, anelectrochemical test strip800, which may be used for measuring glucose concentration in bodily fluids such as blood or interstitial fluid, includes a first workingelectrode808, asecond working electrode806, and areference electrode810. Anactive reagent layer820 is disposed on first workingelectrode808 andreference electrode810 whereactive reagent layer820 completely covers first workingelectrode808 and at least partially coversreference electrode810. Aninactive reagent layer818 is disposed on second workingelectrode806.
• In an embodiment of this invention,active reagent layer820 may include, for example, glucose oxidase and a mediator such as, for example, ferricyanide.Inactive reagent layer818 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 workingelectrode808 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 workingelectrode806 will also be a superposition of oxidation current sources similar to first workingelectrode808, but the first, desirable, oxidation current should not occur because there is no enzyme present on second workingelectrode806. Because the oxidation current measured at second workingelectrode806 depends only on interferents, and the oxidation current measured at first workingelectrode808 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 workingelectrode808 and second workingelectrode806. In such a case, the current density of first workingelectrode808 is subtracted from the current density of the second workingelectrode806 to calculate a corrected glucose current density G where
  G=W_E−WE₂  (Eq 8)
  where WE₁is the current density at first workingelectrode808 and WE₂is the current density at second workingelectrode806.
• In an alternative embodiment to this invention, the interferent oxidation current density at second workingelectrode806 may be slightly different than the interferent oxidation current density at first workingelectrode808 because there is no enzyme on second workingelectrode806. 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 describedEquation 8.
  G=WE₁−(K×WE₂)   (Eq 9)
  where K can range from about 0.5 to about 1.5.
• Test strip800 includes asubstrate50, aconductive layer802, aninsulation layer804,inactive reagent layer818,active reagent layer820, anadhesive layer830, and atop layer824.Test strip800 may be manufactured by sequentially printing five layers which areconductive layer802,insulation layer804,inactive reagent layer818,active reagent layer820, andadhesive layer830 ontosubstrate50.Top layer824 may be assembled by a lamination process.Test strip800 further includes afirst side54, asecond side56, adistal portion58, and aproximal portion60.
• In one embodiment of the present invention,substrate50 is an electrically insulating material such as plastic, glass, ceramic, and the like. In an embodiment of this invention,substrate50 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® ST328 which is manufactured by DuPont Teijin Films.Substrate50 may also include an acrylic coating which is applied to one or both sides to improve ink adhesion.
• The first layer deposited onsubstrate50 isconductive layer802 which includes first workingelectrode808, second workingelectrode806,reference electrode810, andstrip detection bar17. 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 inFIG. 1.Conductive layer802 may be disposed onsubstrate50 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 forconductive layer802 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 ofconductive layer802 can vary depending on the desired resistance and conductivity of the printed material.
• Afirst contact814, asecond contact812, and areference contact816 may be used to electrically interface with a meter. This allows the meter to electrically communicate to first workingelectrode808, second workingelectrode806, andreference electrode810 via, respective,first contact814,second contact812, andreference contact816.
• The second layer deposited onsubstrate50 isinsulation layer804.Insulation layer804 is disposed on at least a portion ofconductive layer802 as shown inFIGS. 1 and 2.FIG. 2 is a simplified plane view ofdistal portion58 oftest strip800 which highlights the position of first workingelectrode808, second workingelectrode806, andreference electrode810 with respect toinsulation layer804.Insulation layer804 further includes acutout18 which may have a rectangular shaped structure as shown inFIG. 1 and2.Cutout18 exposes a portions of first workingelectrode808, second workingelectrode806, andreference electrode810 which can be wetted with liquid.Cutout18 includes a cutout width W20 and a cutout length L26. Cutout width W20 corresponds to a width of second workingelectrode806,reference electrode810, and first workingelectrode808 as illustrated inFIG. 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.
• In one embodiment of the present invention, second workingelectrode806 and first workingelectrode808 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 electrode810 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 workingelectrode806 andreference electrode810; and betweenreference electrode810 and first workingelectrode808 which may range from about 0.2 mm to about 0.6 mm.
• In an alternative embodiment of the present invention, an area of first workingelectrode808 may be different than an area of second workingelectrode806. A ratio of first workingelectrode808 area:second workingelectrode806 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 workingelectrode806. The area of second workingelectrode806 may be increased by modifying the geometry of acutout6008 as shown inFIG. 20.
• FIG. 2 shows that strip800 may be cut along incision line A-A′ after it is fully laminated as illustrated inFIG. 1. In the process of cuttingtest strip800 along incision line A-A′ as illustrated inFIG. 1, asample inlet52 is created in which a liquid sample can be applied fordosing test strip800.
• FIGS.3 to5 are a simplified plane view ofdistal portion58 oftest strip800 according to the embodiment of the present invention illustrated inFIG. 1, which show various positions ofactive reagent layer820 andinactive reagent layer818 with respect to each other. FIGS.6 to8, which correspond to FIGS.3 to5 respectively, do not showinsulation layer804 to help demonstrate more clearly the relationship between theconductive layer802,active reagent layer820, andinactive reagent layer818.
• Test strip800 may haveinactive reagent layer818 disposed on second workingelectrode806 such that it completely covers second workingelectrode806 as is illustrated in FIGS.3 to5. In one embodiment of this invention,inactive reagent layer818 completely covers second workingelectrode806, but does not touchreference electrode810 as is illustrated inFIGS. 3 and 4. In another embodiment of this invention,inactive reagent layer818 completely covers second workingelectrode806 and at least partially coversreference electrode810 as is illustrated inFIG. 5.
• In an embodiment of this invention,inactive reagent layer818 includes at least an oxidized mediator, such as ferricyanide, and may optionally include an inert protein or inactivated enzyme.Inactive reagent layer818 may further include a citrate buffer atpH 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. Becauseinactive reagent layer818 does not include an active enzyme, the oxidation current measured at second workingelectrode806 is not proportional to the glucose concentration. For this reason, one skilled in the art may refer to second workingelectrode806 as a dummy electrode.
• In an embodiment of this invention, the inert protein or deactivated enzyme ininactive reagent layer818 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.
• Test strip800 hasactive reagent layer820 disposed on first workingelectrode808 as illustrated in FIGS.3 to5. In another embodiment of this invention,active reagent layer820 completely covers first workingelectrode808, but does not touchreference electrode810. In another embodiment of this invention,active reagent layer820 completely covers first workingelectrode808 and at least partially coversreference electrode810 as illustrated in FIGS.3 to5.
• In an embodiment of this invention,active reagent layer820 includes at least an oxidized mediator, and an enzyme.Active reagent layer820 may further include a citrate buffer atpH 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. Becauseactive reagent layer820 does include the enzyme, the oxidation current measured at first workingelectrode808 is proportional to the glucose concentration.
• It should be noted that if screen printing were used for depositing bothinactive reagent layer818 andactive reagent layer820, 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 hasinactive reagent layer818 disposed on second workingelectrode806, andactive reagent layer820 disposed on first workingelectrode808 andreference electrode810. In this embodiment,inactive reagent layer818 does not touch or overlap withactive reagent layer820. Because the area of second workingelectrode806, first workingelectrode808 andreference electrode810 is relatively small, it can be difficult to sequentially align and coatinactive reagent layer818 andactive reagent layer820, respectively, with the desired yield. It should also be noted that relatively small electrode areas (e.g. about 0.6 mm²) are preferred because this allows the volume of liquid sample required for a test strip to be small.
• In an embodiment of this invention,inactive reagent layer818 is printed first and then dried at an elevated temperature.Active reagent layer820 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. Becauseactive reagent layer820 is deposited second, it is exposed to only one drying step as opposed to the two drying steps forinactive reagent layer818. This helps stabilize both mediator and enzyme withinactive reagent layer820 because under certain conditions enzymes can degrade with continued exposure to elevated temperatures.
• In an embodiment of this invention,FIG. 4 showsinactive reagent layer818 disposed on second workingelectrode806, andactive reagent layer820 disposed on first workingelectrode808 andreference electrode810. In this embodimentinactive reagent layer818 andactive reagent layer820 are immediately adjacent to each other. In such a case, theinactive reagent layer818 andactive reagent layer820 would touch, but typically not overlap with each other to any significant extent. Although the printing process targets the alignment such thatinactive reagent layer818 andactive reagent layer820 are immediately adjacent to each other, normal manufacturing variation will cause some overlap to occur with a certain frequency betweeninactive reagent layer818 andactive reagent layer820. Likewise, such variation will also causeinactive reagent layer818 to sometimes not touchactive reagent layer820. Becauseinactive reagent layer818 was allowed to touch or not touchactive reagent layer820 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.
• It should be noted that the overlap ofinactive reagent layer818 withactive reagent layer820 does not affect the glucose measurement as long as the enzyme fromactive reagent layer820 cannot diffuse, to any significant extent in the time allowed for the measurement (i.e. about 5 seconds or less), to second workingelectrode806. If enzyme were to diffuse to second workingelectrode806, then first workingelectrode808 would measure a glucose current in addition to the non-enzyme specific currents. This would preventtest strip800 from effectively reducing the background signal.
• It should also be noted that if the overlap ofinactive reagent layer818 withactive reagent layer820 were to occur onreference electrode810 that this would not affect the glucose measurement. In such a case, the amount of enzyme and/or oxidized mediator onreference electrode810 will increase, but should not affect the glucose measurement or the background correction algorithm.
• Yet another embodiment of this invention which improves upon the method of coatinginactive reagent layer818 andactive reagent layer820 is shown inFIG. 5Inactive reagent layer818 may be coated such that it completely covers second workingelectrode806 and a portion ofreference electrode810. Similarly,active reagent layer820 may be coated such that it completely covers first workingelectrode808 and at least a portion ofreference electrode810. In an embodiment of this invention, the printing process can target the alignment such thatinactive reagent layer818 andactive reagent layer820 substantially overlap with each other onreference electrode810 at anoverlap zone822. In such a case,inactive reagent layer818 andactive reagent layer820 may mix with each other atoverlap zone822. Because the length of bothinactive reagent layer818 andactive reagent layer820 was further increased compared to the embodiment described inFIG. 4, the alignment and coating ofactive reagent layer820 andinactive reagent layer818 to first workingelectrode808 and second workingelectrode806 was yet further improved.
• It should be noted that second working electrode806 (e.g. dummy electrode) is located ondistal portion58 oftest strip800 as illustrated in FIGS.1 to5. This causes the physiological fluid to sequentially wet in the following order—second workingelectrode806,reference electrode810, and then first workingelectrode808.Test strip800 was purposefully designed to have inactive reagent layer818 (which contains no enzyme) upstream of active reagent layer820 (which does contain enzyme). This reduces the possibility of enzyme being present at both second workingelectrode806 and first workingelectrode808. Ifactive reagent layer820, which contains enzyme, was coated over second workingelectrode806, and no enzyme were present over first workingelectrode808 then it would be possible that some enzyme could be swept to first workingelectrode808 from second workingelectrode806. The presence of a significant amount of enzyme on first workingelectrode808 would prevent the background signal from being reduced through the use of the dummy electrode format.
• In an embodiment of this invention,top layer824 may be in the form of anintegrated lance826 as shown inFIG. 1. In such an embodiment,top layer824 may include alance826 which is located atdistal portion58.Lance826, which may also be referred to as a penetration member, may be adapted to pierce a user's skin and draw blood intotest strip800 such that second workingelectrode806, first workingelectrode808, andreference electrode810 are wetted.Top layer824 is adhered totest strip800 byadhesive layer830. Thisadhesive layer830 can be a heat seal or a pressure sensitive adhesive.Lance826 includes alancet base832 that terminates atdistal portion58 of assembledtest strip800.Lance826 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 whichtop layer824 is conductive,top layer824 may also be used as areference electrode810 which is orientated with a facing relationship to second workingelectrode806 and first workingelectrode808. 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,lance826 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.
• In an embodiment of the present invention,adhesive layer830 has a height of about 70 to 110 microns.Adhesive layer830 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 layer830 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).
• In a method of this invention, the background variations are reduced by subtracting a first current from first workingelectrode808 from a second current from second workingelectrode806. To initiate a test, a sample is applied to sampleinlet52 which allows a current to be measured at second workingelectrode806 and first workingelectrode808. Because second workingelectrode806 does not have a glucose oxidizing enzyme disposed thereon, a magnitude of an oxidation current at second workingelectrode806 is proportional to an amount of interfering compounds present ontest strip800 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 workingelectrode808 and second workingelectrode806 to reduce the effects of interfering compounds present in the sample and also for interfering compounds that may be present ontest strip800.
• FIG. 9 is a simplified schematic showing ameter900 interfacing withtest strip800.Meter900 has at least three electrical contacts that form an electrical connection to second workingelectrode806, first workingelectrode808, andreference electrode810. In particularsecond contact812 andreference contact816 connect tofirst voltage source910;first contact814 andreference contact816 connect tosecond voltage source920. When performing a test,first voltage source910 applies a first potential E1 between second workingelectrode806 andreference electrode810; andsecond voltage source920 applies a second potential E2 between first workingelectrode808 andreference electrode810.
• 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 workingelectrode806, first workingelectrode808, andreference electrode810 are covered with blood. This allows second workingelectrode806 and first workingelectrode808 to measure a current which is proportional to glucose and/or non-enzyme specific sources. After about 5 seconds from the sample application,meter900 measures an oxidation current for both second workingelectrode806 and first workingelectrode808.
• FIG. 10 is a simplified schematic showing ameter900 interfacing withtest strip800. In contrast toFIG. 9,top layer824 is conductive and used as a reference electrode instead ofreference electrode810 which is disposed onsubstrate50. More particularly,FIG. 10 shows thattop layer824, in the form of a reference electrode, has a facing relationship with first workingelectrode808 and second workingelectrode806. In this case,meter900 forms an electrical contact totop layer824 instead of atreference contact816 as is shown inFIG. 1.
• FIG. 21 is an exploded perspective view of atest strip1000 according to another embodiment of the present invention. The oxidation current measured at a first workingelectrode100 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 workingelectrode100 and indirectly through a mediated mechanism via a mediator.
• Second workingelectrode102 has a geometric trace that has anactive portion102awhich is coated withactive reagent820 and aninactive portion102iwhich is coated withinactive reagent818 as illustrated in FIGS.22 to27. The oxidation current sources measured atactive portion102awill be similar to first workingelectrode100.Inactive portion102iof second workingelectrode102 will oxidize interferents and not oxidize glucose because there is no enzyme present. Further,inactive portion102iwill oxidize interferents directly at second workingelectrode102 and indirectly through a mediated mechanism via a mediator. Because the oxidation current measured atinactive portion102idoes not depend on glucose and the area ofinactive portion102iis known, it is possible to calculate its contribution to the interferent oxidation current measured at second workingelectrode102. In turn, using the interferent oxidation current calculated forinactive portion102iand knowing the area of first workingelectrode100 and the area ofactive portion102a,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 portion102ihelps correct the glucose current for direct and mediated interference oxidation. It should also be noted thatinactive portion102iandactive portion102amay sometimes by referred to as an inactive region and an active region, respectively.
• An algorithm may, therefore be used to calculate a corrected glucose current that is independent of interferences. After dosing a sample ontotest strip1000, a constant potential is applied to first workingelectrode100 and second workingelectrode102 and a current is measured for both electrodes. At first workingelectrode100 whereactive reagent layer820 covers the entire electrode area, the following equation can be used to describe the components contributing to the oxidation current,
  WE₁=G+I_1a  (Eq 1)
  where WE₁is a current density at the first working electrode, G is a current density due to glucose which is independent of interferences, and I_1ais a current density due to interferences oxidized at first workingelectrode100 which is covered withactive reagent820.
• At second workingelectrode102 which is partially covered withactive reagent820 andinactive reagent818, the following equation can be used to describe the components contributing to the oxidation current,
  WE₂=G+I_2a+I_2i  (Eq 2)
  where WE₂is a current density at the second working electrode, I_2ais a current density due to interferences at theactive portion102a,and I²ⁱis a current density due to interferences atinactive portion102i.
• To reduce the effects of interferences, an equation is formulated which describes the relationship between the interferent current atactive portion102aandinactive portion102i.It is approximated that the interferent oxidation current density measured atactive portion102ais the same as the current density measured at theinactive portion102i.This relationship is further described by the following equation,$\begin{array}{cc}{I}_{2a}=\frac{{\mathrm{<figure-callout\; label="A"\; filenames="US20050139469A1-20050630-D00010.png,US20050139469A1-20050630-D00011.png"\; state="\{\{state\}\}">A</figure-callout>}}_{2a}}{{\mathrm{<figure-callout\; label="A"\; filenames="US20050139469A1-20050630-D00010.png,US20050139469A1-20050630-D00011.png"\; state="\{\{state\}\}">A</figure-callout>}}_{2i}}\times I_{2i}& \left(\mathrm{Eq}3a\right)\end{array}$
  where A_2ais an area of second working electrode covered withactive reagent layer820 and A_2iis an area of second working electrode covered withinactive reagent layer818.
• Inactive portion102ican oxidize interferents, but not glucose because it is not coated with enzyme.Active portion102acan oxidize glucose and interferents. Because it was experimentally found thatinactive portion102ioxidizes interferents in a manner proportional to the area ofactive portion102a,it is possible to predict the proportion of interferent current measured overall at second workingelectrode102. This allows the overall current measured at second working electrode102 (i.e. WE₂) to be corrected by subtracting the contribution of the interferent current. In an embodiment of the present invention the ratio of A_2i:A_2amay 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.
• In an alternative embodiment of the present invention, I_2amay be different than I_2i. This may be ascribed to a more efficient or less efficient oxidation of interferents at theactive portion102abecause 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_2a=f×I_2i  (Eq 3b)
  where f is a correction factor which incorporates the effects of the interferent oxidation efficiency of theactive portion102ato inactive portion120i.
• 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 have4 unknowns which are G, I_2i, I_2a, and I_1a. However, I_1aand I_2acan be conservatively assumed to be equal because they are measured at the same conductive material and coated with the sameactive reagent layer820.Equation 1 can be rearranged to the following form.
  G=WE₁−I_1a=WE₁−I_2a  (Eq 4)
  Next, I_2afrom Equation 3a can be substituted intoEquation 4 to yieldEquation 5.$\begin{array}{cc}G={\mathrm{WE}}_l-\left[\frac{{\mathrm{<figure-callout\; label="A"\; filenames="US20050139469A1-20050630-D00010.png,US20050139469A1-20050630-D00011.png"\; state="\{\{state\}\}">A</figure-callout>}}_{2a}}{{\mathrm{<figure-callout\; label="A"\; filenames="US20050139469A1-20050630-D00010.png,US20050139469A1-20050630-D00011.png"\; state="\{\{state\}\}">A</figure-callout>}}_{2i}}\times I_{2i}\right]& \left(\mathrm{Eq}5\right)\end{array}$
  Next,Equation 1 andEquation 2 can be combined to yieldEquation 6.
  I_2i=WE₂−WE₁  (Eq 6)
  Next, I_2ifromEquation 6 can be substituted intoEquation 5 to yield Equation 7a.$\begin{array}{cc}G={\mathrm{WE}}_1-\left\{\left(\frac{A_{2a}}{{\mathrm{<figure-callout\; label="A"\; filenames="US20050139469A1-20050630-D00010.png,US20050139469A1-20050630-D00011.png"\; state="\{\{state\}\}">A</figure-callout>}}_{2i}}\right)X\left({\mathrm{WE}}_2-{\mathrm{WE}}_1\right)\right\}& \left(\mathrm{Eq}7a\right)\end{array}$
• Equation 7a outputs a corrected glucose current density G which removes the effects of interferences requiring only the measured current density from first workingelectrode100 and second working electrode102 (i.e. WE₁and WE₂), 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$\left(i.e\frac{A_{2a}}{{\mathrm{<figure-callout\; label="A"\; filenames="US20050139469A1-20050630-D00010.png,US20050139469A1-20050630-D00011.png"\; state="\{\{state\}\}">A</figure-callout>}}_{2i}}\right).$
  In one embodiment of the present invention the proportion$\frac{A_{2a}}{{\mathrm{<figure-callout\; label="A"\; filenames="US20050139469A1-20050630-D00010.png,US20050139469A1-20050630-D00011.png"\; state="\{\{state\}\}">A</figure-callout>}}_{2i}}$
  may be programmed into a glucose meter, in, for example, a read only memory. In another embodiment of the present invention, the proportion$\frac{A_{2a}}{{\mathrm{<figure-callout\; label="A"\; filenames="US20050139469A1-20050630-D00010.png,US20050139469A1-20050630-D00011.png"\; state="\{\{state\}\}">A</figure-callout>}}_{2i}}$
  may be transferred to the meter via a calibration code chip which would may account for manufacturing variations in A_2aor A_2i.
• In an alternative embodiment to thepresent invention Equation 1, 2, and 3b may be used when the interferent oxidation current density foractive portion102ais different from the interferent oxidation current density ofinactive portion102i.In such a case, an alternative correction Equation 7b is derived as shown below.
  G=WE₁{f×(WE₂−WE₁)}  (Eq 7b)
• 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 WE₂is about 10% or greater than WE₁, then the meter would use Equation 7a or 7b to correct for the current output. However, if WE₂is about 10% or less than WE₁, the meter would simple take an average current value between WE₁and WE₂to 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 WE₂is sufficiently greater than WE₁(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.
• 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 strip1000 includes asubstrate50, aconductive layer164, aninsulation layer106, aninactive reagent layer818, anactive reagent layer820, anadhesive layer830, and atop layer824.Test strip1000 further includes adistal end58 and aproximal end60. It should be noted thattest strip1000 is a modification oftest strip800 so that anactive reagent coating820 covers a portion of both a first workingelectrode100 and asecond working electrode102. This allows for two glucose measurements to be made while at the same time allows for the correction of interferents which develop withintest strip1000 or are dosed intotest strip1000.Test strip1000 would employ either Equation 7a or 7b for reducing the effect of interfering compounds or increased background. In contrast totest strip800,test strip1000 has a modification toconductive layer164 andinsulation layer106.Substrate50,inactive reagent layer818,active reagent layer820,adhesive layer830 andtop layer824 are similar in both shape and material for bothtest strip1000 andtest strip800.
• FIGS.22 to24 are a simplified plane view ofdistal portion58 oftest strip1000, according to the embodiment of the present invention illustrated inFIG. 21, which show various positions ofactive reagent layer820 andinactive reagent layer818 with respect to each other. FIGS.25 to27, which correspond to FIGS.22 to24 respectively, do not showinsulation layer804 to help demonstrate more clearly the relationship between theconductive layer164,active reagent layer820, andinactive reagent layer818.
• Intest strip1000,conductive layer164 is disposed onsubstrate50.Conductive layer164 includes a first workingelectrode100, asecond working electrode102, areference electrode104, afirst contact101, asecond contact103, areference contact105, astrip detection bar17, as shown inFIG. 21. In contrast totest strip800, second workingelectrode806 and first workingelectrode102 has a C-shape.
• FIG. 22 is a simplified plane view of first workingelectrode100, second workingelectrode102, andreference electrode104,insulation layer106,inactive reagent layer818, andactive reagent layer820.Insulation layer106 includes acutout108 which defines the area of second workingelectrode102 to have aninactive portion102iand anactive portion102a.In this embodiment,inactive reagent layer818 was disposed oninactive portion102iandactive reagent layer820 was disposed onactive portion102a,first workingelectrode100, andreference electrode104.FIG. 22 shows thatinactive reagent layer818 does not touch or overlap withactive reagent layer820.
• Test strip1000 differs fromtest strip800 in that bothinactive reagent layer818 andactive reagent layer820 both coat a portion of second workingelectrode102. 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 makingtest strip1000 as shown inFIG. 22 is that it can be difficult to sequentially align and coat the respectiveinactive reagent layer818 andactive reagent layer820 so that they do not touch each other with the desired yield because the area of first workingelectrode100, second workingelectrode102 andreference electrode104 is relatively small.
• In an embodiment of this invention,FIG. 23 showsinactive reagent layer818 disposed oninactive portion102i,andactive reagent layer820 disposed onactive portion102a,first workingelectrode100, andreference electrode104. In this embodimentinactive reagent layer818 andactive reagent layer820 are immediately adjacent to each other. In such an ideal case theinactive reagent layer818 andactive reagent layer820 would touch, but not substantially overlap with each other. Although the printing process targets the alignment such thatinactive reagent layer818 andactive reagent layer820 are immediately adjacent to each other, normal manufacturing variation will cause some overlap to occur with a certain frequency betweeninactive reagent layer818 andactive reagent layer820. Likewise, such variation will also causeinactive reagent layer818 to not touchactive reagent layer820 at a certain frequency. Becauseinactive reagent layer818 was allowed to touch or not touchactive reagent layer820, the yield of acceptable test strips was improved.
• Yet another embodiment of this invention which improves upon the method of coatinginactive reagent layer818 andactive reagent layer820 is shown inFIG. 24.Inactive reagent layer818 may be coated such that it completely coversinactive portion102iand a portion ofreference electrode104. Similarly,active reagent layer820 may be coated such that it completely coversactive portion102a,first workingelectrode100 and at least a portion ofreference electrode104. In an embodiment of this invention, the printing process can target the alignment such thatinactive reagent layer818 andactive reagent layer820 substantially overlap with each other onreference electrode810 at anoverlap zone822. In such a case,inactive reagent layer818 andactive reagent layer820 may mix with each other atoverlap zone822. Because the length of bothinactive reagent layer818 andactive reagent layer820 was further increased compared to the embodiment described inFIG. 23, the alignment and coating ofactive reagent layer820 andinactive reagent layer818 was yet further improved in terms of manufacturing yield.
• 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
• Test strips800 were prepared as illustrated in FIGS.1 to3a.Test strips800 were tested in blood which were exposed to varying levels of sterilizing radiation. Totest strips800, they were electrically connected to a potentiostat which has the means to apply a constant potential of +0.4 volts between first workingelectrode808 andreference electrode810; and second workingelectrode806 and thereference electrode810. A sample of blood is applied to sampleinlet52 allowing the blood to wick into the sample receiving chamber and to wet first workingelectrode808,reference electrode810, and second workingelectrode806.Active layer820 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 layer818 becomes hydrated with blood and does not generate additional ferrocyanide that was not present withininactive layer818 before hydration. After about 5 seconds from the sample application totest strip800, an oxidation of ferrocyanide and/or interferences are measured as a current for both first workingelectrode808 and second workingelectrode806.
EXAMPLE 2
• Two batches of test strips were prepared to show that the use ofinactive reagent layer818 andactive reagent layer820 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 istest strip800 and is referred to asBatch1. The second test strip batch, which is referred to asBatch2, is also similar totest strip800, but does not includeinactive reagent layer818 and also has a modified active reagent layer which covers both first workingelectrode808, second workingelectrode806, andreference electrode810. When testingBatch1, the difference in current from first workingelectrode808 and second workingelectrode806 was used to calculate a corrected signal current which was then converted to a glucose concentration. When testingBatch2, the current from second workingelectrode806 and first workingelectrode808 were summed together to determine a value which was then used to calculate an uncorrected glucose concentration. Before testing with blood, bothBatch1 andBatch2 test strips were treated with 0 kGy and 25 kGy of gamma radiation. Next, the four test cases, which areBatch1—0 kGy,Batch1—25 kGy,Batch2—0 kGy, andBatch2—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.
• FIGS.11 to15 show thatBatch1 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 forBatch1 test strips. This shows that the use ofactive reagent layer820 andinactive reagent layer818 helps compensate for background levels of ferrocyanide produced during the sterilization process.
• FIGS.11 to13 show thatBatch2 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. BecauseBatch2 test strip did not haveinactive reagent layer818, the background reduction method could not be implemented.Batch2 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
• Another batch of test strips, which is referred to asBatch3, was prepared in a manner similar totest strip800 except that second workingelectrode806 was not coated with eitheractive reagent layer820 orinactive reagent layer818. In this example,Batches1 to3 were tested to evaluate the overall accuracy in the presence of interfering compounds such as uric acid and gentisic acid.
• Batch1,Batch2, andBatch3 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 thatBatch1 andBatch3 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,Batch2 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 workingelectrode806 not coated with enzyme allows for an effective correction of the glucose signal in the presence of high concentrations of gentisic acid.
• Batch1,Batch2, andBatch3 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 thatBatch1 andBatch3 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,Batch2 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 workingelectrode806 not coated with enzyme allows for an effective correction of the glucose signal in the presence of high concentrations of uric acid.
• 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.

Claims (6)

1. An electrochemical sensor comprising:

a substrate;

a first working electrode disposed on said substrate;

a second working electrode disposed on said substrate;

a reference electrode;

an active reagent layer disposed on said first working electrode, wherein said active reagent layer completely covers said first working electrode; and

an inactive reagent layer disposed on said second working electrode, wherein said inactive reagent completely covers said second working electrode.

2. An electrochemical sensor according toclaim 1 wherein:

said first working electrode, said second working electrode and said reference electrode are positioned in a sample receiving chamber;

said sample receiving chamber having a proximal and a distal end, said distal end including a first opening which is adapted to receive bodily fluids; and

said second working electrode being positioned adjacent said first opening.

3. An electrochemical sensor according toclaim 2 wherein said first working electrode and said reference electrode are positioned proximal to said second working electrode.

4. An electrochemical sensor comprising:

a substrate;

a first working electrode disposed on said substrate;

a second working electrode disposed on said substrate;

a reference electrode;

an active reagent layer disposed on said first working electrode, wherein said active reagent layer completely covers said first working electrode;

said second working electrode having an active region and an inactive region, said active reagent layer disposed on a active region of said second working electrode and an inactive reagent layer disposed on said inactive region of said second working electrode.

5. An electrochemical sensor according toclaim 4 wherein:

said first working electrode, said second working electrode and said reference electrode are positioned in a sample receiving chamber;

said sample receiving chamber having a proximal and a distal end, said distal end including a first opening which is adapted to receive bodily fluids; and

said inactive region of said second working electrode being positioned adjacent said first opening.

6. An electrochemical sensor according toclaim 5 wherein said active region of said second working electrode and said first working electrode are positioned proximal to said inactive region of said second working electrode.

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