US20050133368A1

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Publication number: US20050133368A1
Application number: US10/976,489
Authority: US
Inventors: Oliver Davies; Robert Marshall; Damian Baskeyfield; Lynsey Whyte; Elaine Leiper
Original assignee: Individual
Current assignee: LifeScan Scotland Ltd
Priority date: 2003-10-31
Filing date: 2004-10-29
Publication date: 2005-06-23
Also published as: EP1678490B1; CA2543957C; HK1093095A1; HK1091896A1; WO2005045413A1; US20050183965A1; DE602004021835D1; ATE460661T1; EP1678491B1; JP2007514931A; CA2551058A1; ATE360816T1; AU2004288004A1; EP1678491A1; JP2007514928A; KR101201245B1; AU2004288004B2; SG131941A1; KR101179998B1; US7618522B2

Abstract

This invention describes an electrochemical sensor which is adapted to reduce the effects of interfering compounds in bodily fluids when measuring an analyte in such fluids using an electrochemical strip. The sensor includes a substrate, a first and second working electrodes, and a reference electrode. A reagent layer is disposed on the electrodes such that, in one embodiment it completely covers all of the first working electrode, but only partially covers the second working electrode and, in a second embodiment, it only covers a portion of the first and the second working electrode. The portion of the working electrodes not covered by the reagent layer and is used to correct for the interference effect on the analyte measurement.

Description

PRIORITY

The present invention claims priority to the following U.S. 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

The present invention is related to the following co-pending U.S. 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-5065], filed on Oct. 29, 2004; U.S. patent application Ser. No. ______ [Attorney Docket Number DDI-5066], filed on Oct. 29, 2004; and U.S. patent application Ser. No. ______ [Attorney Docket Number DDI-5067], filed on Oct. 29, 2004.

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 in a glucose monitoring system wherein the electrochemical strip has electrodes with uncoated regions.

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 invention described herein is directed to an electrochemical sensor which reduces the effects of interferences. An electrochemical sensor according to the present invention includes a substrate, at least first and second working electrodes and a reference electrode. In one embodiment of an electrochemical sensor according to the present invention, a reagent layer is disposed on the electrodes such that it completely covers all of the first working electrode and only partially covers the second working electrode. In a method according to the present invention, the oxidation current generated at the portion of the second working electrode not covered by the reagent layer is used to correct for the effect of interfering substances on the glucose measurement.

In one embodiment of the present invention, the electrochemical glucose test strip includes a first and second working electrodes, where the first working electrode is completely covered with a reagent layer and the second working electrode is only partially covered with the reagent layer. Thus, the second working electrode has a reagent coated area and an uncoated area. The reagent layer may include, for example, a redox enzyme such as glucose oxidase and a mediator such as, for example, ferricyanide. The first working electrode will have a superposition of two oxidation current sources, one from glucose and a second from interferents. Similarly, the second working electrode will have a superposition of three oxidation current sources from glucose, interferents at the reagent coated portion, and interferents at the uncoated portion. The uncoated portion of the second working electrode will only oxidize interferents and not oxidize glucose because there is no reagent is in this area. The oxidation current measured at the uncoated portion of the second working electrode may then be used to estimate the total interferent oxidation current and calculate a corrected oxidation current which removes the effects of interferences.

In an alternative strip embodiment according to the present invention, the electrochemical glucose test strip includes a first and second working electrodes, where the first and second working electrode are only partially covered with the reagent layer. Thus, in this embodiment both the first and second working electrode have a reagent coated portion and an uncoated portion. The first uncovered area of the first working electrode and the second uncovered area of the second working electrode are different. The oxidation current measured at the uncoated portion of the first and second working electrodes are used to estimate the interferent oxidation current for the uncoated portion and to calculate a corrected glucose current.

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 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 a reagent layer is illustrated with the conductive layer and the insulation layer;

FIG. 4 is an exploded perspective view of a test strip according to a further embodiment of the present invention;

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. 4 including of a conductive layer and an insulation layer; and

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. 4 wherein a reagent layer is illustrated with the conductive layer and the insulation 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. 4 wherein a reagent layer is illustrated with the conductive layer.

FIG. 8 is a simplified plane view of a distal portion of a test strip according to another embodiment of the present invention wherein a reagent layer is illustrated with the conductive layer that helps reduce an IR drop effect.

FIG. 9 is a simplified plane view of a distal portion of a test strip according to yet another embodiment of the present invention wherein a reagent layer is illustrated with the conductive layer and the insulation layer such there are two working electrodes that have an uncoated portion.

FIG. 10 is a simplified plane view of a distal portion of a test strip according to still yet another embodiment of the present invention wherein a reagent layer is illustrated with the conductive layer and the insulation layer such there are two working electrodes that have an uncoated portion.

FIG. 11 is a graph showing the current at a first working electrode of a strip designed in accordance with the present invention tested with 70 mg/dL glucose samples in blood spiked with varying levels of uric acid.

FIG. 12 is a graph showing the current at a first working electrode at a strip designed in accordance with the present invention tested with 240 mg/dL glucose samples in blood spiked with varying levels of uric acid.

FIG. 13 is an exploded perspective view of a test strip that has an integrated lance.

FIG. 14 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.

DETAILED DESCRIPTION OF THE INVENTION

This invention described herein includes a test strip and method for improving the selectivity of an electrochemical glucose measuring system.

FIG. 1 is an exploded perspective view of atest strip62 according to a first embodiment of the present invention.Test strip62, as illustrated inFIG. 1, may be manufactured by a series of 6 consecutive printing steps which lay down six layers of material onsubstrate50. The six layers may be deposited by, for example, screen printing onsubstrate50. In an embodiment of this invention, the 6 layers may include aconductive layer64, aninsulation layer16, areagent layer22, anadhesive layer66, ahydrophilic layer68, and atop layer40.Conductive layer64 may further includes first workingelectrode10, second workingelectrode12,reference electrode14,first contact11,second contact13,reference contact15, andstrip detection bar17.Insulation layer16 may further includecutout18.Adhesive layer66 may further include firstadhesive pad24, secondadhesive pad26, and thirdadhesive pads28.Hydrophilic layer68 may further include firsthydrophilic film32, and secondhydrophilic film34.Top layer40 may further includes aclear portion36 andopaque portion38.Test strip62 has afirst side54 andsecond side56, adistal electrode side58, and aproximal electrode side60 as illustrated inFIG. 1. The following sections will describe the respective layers oftest strip62 in more detail.

In one embodiment of the present invention,substrate50 is an electrically insulating material such as plastic, glass, ceramic, and the like. In a preferred 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 layer64 which includes first workingelectrode10, second workingelectrode12,reference electrode14, 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.Reference electrode14 may also be a counter electrode, a reference/counter electrode, or a quasi-reference electrode.Conductive layer64 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 layer64 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 the conductive layer can vary depending on the desired resistance of the conductive layer and the conductivity of the material used for printing the conductive layer.

First contact

11,second contact13, andreference contact15 may be used to electrically interface with a meter. This allows the meter to electrically communicate to first workingelectrode10, second workingelectrode12, andreference electrode14 via, respective,first contact11,second contact13, andreference contact15.

The second layer deposited onsubstrate50 isinsulation layer16.Insulation layer16 is disposed on at least a portion ofconductive layer64 as shown inFIG. 1.FIG. 2 is a simplified plane view of a distal portion oftest strip62 which highlights the position of first workingelectrode10, second workingelectrode12, andreference electrode14 with respect toinsulation layer16.Insulation layer16 further includes acutout18 which may have a T-shaped structure as shown inFIGS. 1 and 2.Cutout18 exposes a portions of first workingelectrode10, second workingelectrode12, andreference electrode14 which can be wetted with liquid.Cutout18 further includes a distal cutout width W1, proximal cutout width W2, a distal cutout length L4 and a proximal cutout length L5. Distal cutout width W1 corresponds to the width of first workingelectrode10 andreference electrode14 as illustrated inFIG. 2. Distal cutout length L4 corresponds to a length which is greater than both first workingelectrode10 andreference electrode14 together. Proximal cutout width W2 and proximal cutout length LS form a rectangular section which exposes the width and length of second workingelectrode12. In accordance with the present invention, distal cutout width W1, proximal cutout width W2, distal cutout length L4 and proximal cutout length L5 may have a respective dimension of approximately 0.7, 1.9, 3.2, and 0.43 mm. In one embodiment of the present invention, first workingelectrode10,reference electrode14, and second workingelectrode12 have a respective length of L1, L2, and L3 which may be about 0.8, 1.6, and 0.4 mm. In accordance with the present invention, electrode spacing S1 is a distance between first workingelectrode10 andreference electrode14; and betweenreference electrode14 and second workingelectrode12 which may be about 0.4 mm.

The third layer deposited onsubstrate50 is areagent layer22.Reagent layer22 is disposed on at least a portion ofconductive layer64 andinsulation layer16 as shown inFIGS. 1.FIG. 3 is a simplified plane view of a distal portion oftest strip62 according to the first embodiment of the present invention which highlights the position ofreagent layer22 with respect to first workingelectrode10, second workingelectrode12,reference electrode14, andinsulation layer16.Reagent layer22 may be in the shape of a rectangle having a reagent width W3 and a reagent length L6 as illustrated inFIGS. 1 and 3. In one embodiment of the invention, reagent width W3 may be about 1.3 mm and reagent length L6 may be about 4.7 mm. In a further embodiment of the present invention,reagent layer22 has a sufficiently large width W3 and length L6 such thatreagent layer22 completely covers first workingelectrode10 andreference electrode14. However,reagent layer22 has an appropriately sized width W3 and length L6 such that second working electrode is not completely covered withreagent layer22. In such a scenario, second workingelectrode12 has a coatedportion12cand anuncoated portions12uas illustrated inFIG. 3.Uncoated portions12umay be in the shape of two rectangles whereuncoated portions12uhas a wing width W4 and a length that corresponds to second working electrode length L3. As a non-limiting example, wing width W4 may be about 0.3 mm. In one embodiment of the present invention,reagent layer22 may include a redox enzyme such as, for example, glucose oxidase or PQQ-glucose dehydrogenase (where PQQ is an acronym for pyrrolo-quinoline-quinone) and a mediator such as, for example, ferricyanide.

The fourth layer deposited onsubstrate50 is anadhesive layer66 which includes a firstadhesive pad24, a secondadhesive pad26, and a thirdadhesive pad28. Firstadhesive pad24 and secondadhesive pad26 form the walls of a sample receiving chamber. In one embodiment of the present invention, firstadhesive pad24 and secondadhesive pad26 may be disposed onsubstrate50 such that neither of the adhesive pads touchesreagent layer22. In another embodiments of the present invention where the strip volume needs to be reduced, firstadhesive pad24 and/or secondadhesive pad26 may be disposed onsubstrate50 such there is overlap withreagent layer22. In an embodiment of the present invention,adhesive layer66 has a height of about 70 to 110 microns.Adhesive layer66 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 layer66 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).

The fifth layer deposited onsubstrate50 is ahydrophilic layer68 which includes a firsthydrophilic film32 and secondhydrophilic film34 as illustrated inFIG. 1.Hydrophilic layer68 forms the “roof” of the sample receiving chamber. The “side walls” and “floor” of the sample receiving chamber are formed by a portion ofadhesive layer66 andsubstrate50, respectively. As a non-limiting example,hydrophilic layer68 may be an optically transparent polyester with a hydrophilic anti-fog coating such as those commercially obtained from 3M. The hydrophilic nature of the coating is used in the design ofstrip62 because it facilitates filling of liquid into the sample receiving chamber.

The sixth and final layer deposited onsubstrate50 is atop layer40 which includes aclear portion36 andopaque portion38 as illustrated inFIG. 1. In accordance with the present invention,top layer40 includes a polyester which is coated on one side with a pressure sensitive adhesive.Top layer40 has anopaque portion38 which helps the user observe a high degree of contrast when blood is underneathclear portion36. This allows a user to visually confirm that the sample receiving chamber is sufficiently filled. Afterstrip62 is fully laminated, it is cut along incision line A-A′ and in the process createssample inlet52 as illustrated inFIG. 3.

The first test strip embodiment as illustrated inFIGS. 1-3 may have a possible drawback in thatreagent layer22 may dissolve in a liquid sample and move a portion of the dissolved reagent layer over theuncoated portions12uof second workingelectrode12. If such a scenario were to occur,uncoated portions12uwould also measure an oxidation current that is also proportional to the glucose concentration. This would degrade the ability to use mathematical algorithms for removing the effect of interferent oxidation. In an alternative embodiment of the present invention,reagent layer22 should be designed to dissolve in such a way that it does not migrate touncoated portions12u. For example,reagent layer22 may be chemically bound to the first workingelectrode10, second workingelectrode12, andreference electrode14 or may have a thickening agent that minimizes the migration of dissolvedreagent layer22.

A further embodiment of the present invention as illustrated inFIG. 4, the embodiment illustrated inFIG. 4 reduces, and in certain circumstances minimizes, the immigration of dissolved reagent to an uncoated portion of the second working electrode. In this embodiment, second workingelectrode102 has a C-shaped geometry where 2 discrete portions of second workingelectrode102 are exposed bycutout108 as illustrated inFIG. 4. In accordance with the present invention,reagent layer110 is disposed on only a portion of second workingelectrode102 to form anuncoated portion102uandcoated portion102cas illustrated inFIG. 6.Uncoated portion102uis adjacent to sampleinlet52.Coated portion102cis adjacent to first workingelectrode100. When applying liquid to sampleinlet52 of an assembledtest strip162, the liquid will flow fromsample inlet52 tocoated portion102cuntil all electrodes are covered with liquid. By positioninguncoated portion102uupstream of the liquid flow, this almost entirely preventsreagent layer110 from dissolving and migrating touncoated portion102u. This enables the mathematical algorithm to accurately remove the effects of interferents from the measured oxidation current.

FIG. 4 is an exploded perspective view of atest strip162.Test strip162 is manufactured in a manner similar totest strip62 except that there are geometric or positional changes to aconductive layer164, aninsulation layer106, and areagent layer110. For the second embodiment of this invention,substrate50,adhesive layer66,hydrophilic layer68, andtop layer40 are the same as the first strip embodiment.Test strip162 has afirst side54 andsecond side56, adistal electrode side58, and aproximal electrode side60. It should also be noted that the first and second test strip embodiment of the present invention may have elements with similar structure which are denoted with the same element number and name. If analogous elements between the respective test strip embodiments are different in structure, the elements may have the same name, but be denoted with a different element number. The following sections will describe the respective layers oftest strip162 in more detail.

For the strip embodiment illustrated inFIG. 4, the first layer deposited onsubstrate50 isconductive layer164 which includes first workingelectrode100, second workingelectrode102,reference electrode104,first contact101,second contact103, andreference contact105, 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. 4.First contact101,second contact103, andreference contact105 may be used to electrically interface with a meter. This allows the meter to electrically communicate to first workingelectrode100, second workingelectrode102, andreference electrode104 via, respective,first contact101,second contact103, andreference contact105.

The second layer deposited onsubstrate50 inFIG. 4 isinsulation layer106.Insulation layer106 is disposed on at least a portion ofconductive layer164 as shown inFIG. 4.FIG. 5 is a simplified plane view of a distal portion oftest strip162 which highlights the position of first workingelectrode100, second workingelectrode102, andreference electrode104 with respect toinsulation layer106.

The third layer deposited onsubstrate50 inFIG. 4 is areagent layer110 such thatreagent layer110 is disposed on at least a portion ofconductive layer164 andinsulation layer106 as shown inFIG. 6.FIG. 6 is a simplified plane view of a distal portion oftest strip162 according to the second embodiment of the present invention which highlights the position ofreagent layer110 with respect to first workingelectrode100, second workingelectrode102,reference electrode104, andinsulation layer106.Reagent layer110 may be in the shape of a rectangle having a reagent width W13 and a reagent length L16. In one embodiment of this invention, reagent width W13 may be about 1.3 mm and reagent length L16 may be about 3.2 mm. In a preferred embodiment of the present invention,reagent layer110 has a sufficient width W13 and length L16 such thatreagent layer110 completely covers first workingelectrode100,coated portion102c, andreference electrode104, but does not coveruncoated portion102u.

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. 4 wherein a reagent layer is illustrated with the conductive layer. In contrast toFIG. 6,FIG. 7 does not showinsulation layer106. This helps demonstrate the conductive relationship betweenuncoated portion102uandcoated portion102cwhich was hidden underneath the opaque character ofinsulation layer106.

For the strip embodiment illustrated inFIG. 4,insulation layer106 is used to define the width of the first workingelectrode100, second workingelectrode102, andreference electrode104.Insulation layer106 further includes acutout108 which may have a T-shaped structure as shown inFIG. 4 to6.Cutout108 exposes a portion of first workingelectrode100, second workingelectrode102, andreference electrode104 which can be wetted with liquid.Cutout108 further includes a distal cutout width W11, proximal cutout width W12, a distal cutout length L14 and a proximal cutout length L15 as illustrated inFIG. 5 and6. Distal cutout width W11 corresponds to the width ofuncoated portion102u. Distal cutout length L14 is greater than the lengthuncoated portion102u. Proximal cutout width W12 and proximal cutout length L15 forms a rectangular section which approximately exposes the width and length of first workingelectrode100,reference electrode104, andcoated portion102c.

In accordance with the present invention, distal cutout width W11, proximal cutout width W12, distal cutout length L14 and proximal cutout length L15 may have a respective dimension of approximately 1.1, 0.7, 2.5, and 2.6 mm.

In the embodiment ofFIG. 4,uncoated portion102u,reference electrode104, first workingelectrode100, andcoated portion102chave a respective length of L10, L12, L11, and L13 which may be about 0.7, 0.7, 0.4, and 0.4 mm. Electrode spacing S11 is a distance betweenuncoated portion102uandreference electrode104 which may be between about 0.2 to 0.75 mm, and more preferably between 0.6 to 0.75 mm. Electrode spacing S10 is a distance betweenreference electrode104 and first workingelectrode100; and betweencoated portion102cand first workingelectrode100 which may be about 0.2 mm. It should be noted that electrode spacing S11 is greater than S10 to decrease the possibility of reagent dissolving and migrating touncoated portion102u. Additionally, electrode spacing S11 is greater than S10 to decrease the possibility ofreagent layer110 being disposed onuncoated portion102ubecause of variations in the printing process. The fourth through sixth layer which is successively disposed onstrip162 in the same manner as the first strip embodiment. The relative position and shape of theadhesive layer66,hydrophilic layer68, andtop layer40 are illustrated inFIG. 4.

In the embodiment of the invention illustrated inFIG. 8, the C-shape of second workingelectrode102 may be partially altered so that the order in which liquid would wet the electrodes would beuncoated portion102u, first workingelectrode100,reference electrode104, and then coatedportion102c. In the alternative format, first workingelectrode100 andcoated portion102cwould be equidistant fromreference electrode104 which is desirable from an IR drop perspective. In the second strip embodiment (i.e. test strip162) illustrated inFIG. 7, the electrodes are arranged so that the order in which liquid would wet the electrodes would beuncoated portion102u,reference electrode104, first workingelectrode100, and then coatedportion102c. Fortest strip162,coated portion102cis farther away fromreference electrode104 than the distance between first workingelectrode100 andreference electrode104.

An algorithm may, therefore be used to calculate a corrected glucose current that is independent of interferences. After dosing a sample onto a test strip, a constant potential is applied to the first and second working electrodes and a current is measured for both electrodes. At the first working electrode where reagent covers the entire electrode area, the following equation can be used to describe the components contributing to the oxidation current,
WE₁=G+I_cov (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_covis a current density due to interferences at the portion of a working electrode covered with reagent.

At the second working electrode which is partially covered with reagent, the following equation can be used to describe the components contributing to the oxidation current,
WE₂=G+I_cov+I_unc (Eq 2)
where WE₂is a current density at the second working electrode and I_uncis a current density due to interferences at the portion of a working electrode not covered with reagent.

To reduce the effects of interferences, an equation is formulated which describes the relationship between the interferent current at the coated portion of the second working electrode and the uncoated portion of the second working electrode. It is approximated that the interferent oxidation current density measured at the coated portion is the same as the current density measured at the uncoated portion. This relationship is further described by the following equation,

\begin{matrix} I_{cov} = \frac{A_{cov}}{A_{unc}} \times I_{unc} & (Eq 3 a) \end{matrix}

where A_covis an area of second working electrode covered with reagent and A_uncis an area of second working electrode not covered with reagent.

Uncoated portions

12ucan oxidize interferents, but not glucose because it is not coated withreagent layer22. In contrast, coatedportion12ccan oxidize glucose and interferents. Because it was experimentally found thatuncoated portions12uoxidizes interferents in a manner proportional to the area ofcoated portion12c, it is possible to predict the proportion of interferent current measured overall at second workingelectrode12. This allows the overall current measured at second workingelectrode12 to be corrected by subtracting the contribution of the interferent current. In an embodiment of the present invention the ratio of A_unc:A_covmay 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 a later section.

In an alternative embodiment of the present invention, the interferent oxidation current density measured at the coated portion may be different than the current density measured at the uncoated portion. This may be ascribed to a more efficient or less efficient oxidation of interferents at the coated portion. In one scenario, the presence of a mediators may enhance the oxidation of interferences relative to the uncoated portion. In another scenario, the presence of viscosity increasing substances such as hydroxyethyl cellulose may decrease the oxidation of interferences relative to the uncoated portion. Depending on the components included in the reagent layer which partially coats the second working electrode, it is possible that the interferent oxidation current density measured at the coated portion may be more or less than the uncoated portion. This behavior may be phenomenologically modeled by re-writing Equation 3a to the following form,
I_cov=f×I_unc (Eq 3b)
where f is a correction factor which incorporates the effects of the interferent oxidation efficiency of the coated to uncoated portion.

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 3 unknowns which are G, I_cov, and I_unc. Equation 1 can be rearranged to the following form.
G=WE₁−I_cov (Eq 4)
Next, I_covfrom Equation 3a can be substituted into Equation 4 to yieldEquation 5.

\begin{matrix} G = {WE}_{1} - [\frac{A_{cov}}{A_{unc}} \times I_{unc}] & (Eq 5) \end{matrix}

Next,Equation 1 and Equation 2 can be combined to yield Equation 6.
I_unc=WE₂−WE₁ (Eq 6)
Next, I_uncfrom Equation 6 can be substituted intoEquation 5 to yield Equation 7a.

\begin{matrix} G = {WE}_{1} - {(\frac{A_{cov}}{A_{unc}}) X ({WE}_{2} - {WE}_{1})} & (Eq 7 a) \end{matrix}

Equation 7a outputs a corrected glucose current density G which removes the effects of interferences requiring only the current density output of the first and second working electrode, and a proportion of the coated to uncoated area of the second working electrode. In one embodiment of the present invention the proportion

\frac{A_{cov}}{A_{unc}}

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_{cov}}{A_{unc}}

may be transferred to the meter via a calibration code chip which would may account for manufacturing variations in A_covor A_unc.

In an alternative embodiment to thepresent invention Equation 1, 2, and 3b may be used when the interferent oxidation current density for the coated portion is different from the interferent oxidation current density of the uncoated portion. 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.

In the embodiment of the present invention illustrated inFIGS. 9 and 10, the first and second working electrodes are partially covered with the reagent layer in such a way that that the uncoated portions of the first and second working electrodes are different. This contrasts the previously described first and second test strip embodiments where the first working electrode is completely covered with the reagent layer.

FIG. 9 is a simplified plane view of a distal portion of atest strip2000 according to yet another embodiment of the present invention wherein areagent layer22 is illustrated with the conductive layer andinsulation layer2002 such there are two working electrodes which have an uncoated portion.Test strip2002 is manufactured in a manner similar totest strip62 except that there is a geometric change to cutout18 as shown inFIG. 1.Test strip2002 has thesame substrate50,conductive layer64,reagent layer22,adhesive layer66,hydrophilic layer68, andtop layer40 astest strip62.Test strip2002 was modified to have acutout2004 which has a dumbbell like shape as illustrated inFIG. 9. The modified shape forcutout2004 allows first workingelectrode2008 to include a firstcoated portion2008cand an firstuncoated portion2008u; and second workingelectrode2006 to include a secondcoated portion2006cand seconduncoated portion2006u. In order fortest strip2000 to effectively reduce the effects of interferents, the two firstuncoated portions2008umust have a different total area than the two seconduncoated portions2006u.

FIG. 10 is a simplified plane view of a distal portion of atest strip5000 according to still yet another embodiment of the present invention wherein areagent layer820 is illustrated with the conductive layer such there are two working electrodes which have an uncoated portion.Test strip5000 is manufactured in a manner similar totest strip162 except that there is a geometric change toconductive layer164 such that both afirst working electrode4002 and asecond working electrode4004 have a C-shape.Test strip5000 has thesame substrate50,insulation layer106,reagent layer110,adhesive layer66,hydrophilic layer68, andtop layer40 astest strip162. The modified geometry allows first workingelectrode4002 to include a firstcoated portion4002cand an firstuncoated portion4002u; and second workingelectrode4004 to include a secondcoated portion4004cand seconduncoated portion4004u. In order fortest strip2000 to effectively reduce the effects of interferents, firstuncoated portion4002umust have a different area than seconduncoated portion4004u.

Test strips

2000 and5000 have an advantage in that they may be easier to manufacture in regards to depositing the reagent layer with the required registration and also any subsequently deposited layers. Furthermore, both the first and second working electrodes will have to some extent the same chemical and electrochemical interactions with any interfering substances thus ensuring greater accuracy in the correction process. With both working electrodes having some level of uncoated area the same reactions will occur on both electrodes but to a different extent. Using a simple modification to Equation 7a, the following Equation 7c can be used as the correction equation for glucose,

\begin{matrix} G = {WE}_{1} - {(\frac{f_{1} + f_{2}}{f_{2} - 1}) \times ({WE}_{2} - {WE}_{1})} & (Eq 7 c) \end{matrix}

where

f_{1} = \frac{A_{cov1}}{A_{unc1}}, f_{2} = \frac{A_{cov1}}{A_{unc2}},

A_unc1=is an uncoated area of the first working electrode, A_unc2=is an uncoated area of the second working electrode, A_cov1=is a coated area of the first working electrode, and A_cov2=is a coated area of the second working electrode.

One advantage of the present invention is the ability to use the first and second working electrode to determine that the sample receiving chamber has been sufficiently filled with liquid. It is an advantage of this invention in that the second working electrode not only corrects the interferent effect, but can also measure. glucose. This allows for a more accurate results because 2 glucose measurements can be averaged together while using only one test strip.

EXAMPLE 1

Test strips were prepared according to the first embodiment of the present invention as illustrated inFIG. 1 to3. These test strips were tested in blood having various concentrations of interferents. To test these strips, they were electrically connected to a potentiostat which has the means to apply a constant potential of 0.4 volts between the first working electrode and the reference electrode; and the second working electrode and the reference electrode. A sample of blood is applied to the sample inlet allowing the blood to wick into the sample receiving chamber and to wet first working electrode, second working electrode, and reference electrode. The reagent layer 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. After about 5 seconds from the sample application to the test strip, an oxidation of ferrocyanide is measured as a current for both the first and second working electrode.

FIG. 11 shows the current responses of the first working electrode tested with 70 mg/dL glucose samples in blood spiked with varying levels of uric acid. The uncorrected current at the first working electrode (depicted by squares) shows an increase in current that is proportional to the uric acid concentration. However, the corrected current (depicted by triangles) which is processed by Equation 7a shows no effect from the increasing uric acid concentration.

FIG. 12 shows the current responses of the first working electrode tested with 240 mg/dL glucose samples in blood spiked with varying levels of uric acid. The purpose of testing strips at 240 mg/dL glucose is to show that the correction algorithm of Equation 7a is also valid over a range of glucose concentrations. Similar toFIG. 11, the uncorrected current at the first working electrode (depicted by squares) shows an increase in current that is proportional to the uric acid concentration. However, the corrected current (depicted by triangles) shows no effect from the increasing uric acid concentration.

EXAMPLE 2

To show that the method of correcting the current for interferents applies to a wide variety of interferents, strips built according to the embodiment ofFIG. 1 were also tested with acetaminophen and gentisic acid at various concentration levels, in addition to uric acid. For purposes of quantitating the magnitude of this effect, a change in glucose output of greater than 10% (for glucose level >70 mg/dL) or 7 mg/dL (for glucose level <=70 mg/dL) was defined as a significant interference. Table 1 shows that the uncorrected current at the first working electrode shows a significant interferent effect at a lower interferent concentration than strips tested with a corrected current response using Equation 7a. This shows that the method of correcting the current output of the first working electrode using Equation 7a is effective in correcting for interferences. Table 1 shows that the current correction in Equation 7a is effective for interferences with respect to acetaminophen, gentisic acid, and uric acid. Table 1 also shows the concentration range of the interferent which is normally found in blood. In addition, Table 1 also shows that the current correction in Equation 7a is effective at 240 mg/dL glucose concentration level.

FIG. 13 shows an exploded perspective view of atest strip800 that is designed to lance a user's skin layer so as cause physiological fluid to be expressed and collected intotest strip800 in a seamless manner.Test strip800 includes asubstrate50, aconductive layer802, aninsulation layer804, areagent layer820, anadhesive layer830, and atop layer824.Test strip800 further includes adistal end58 and aproximal end60.

Intest strip800,conductive layer802 is the first layer disposed onsubstrate50.Conductive layer802 includes asecond working electrode806, a first workingelectrode808, areference electrode810, asecond contact812, afirst contact814, areference contact816, astrip detection bar17, as shown inFIG. 13. The material used forconductive layer802 and the process for printingconductive layer802 is the same for bothtest strip62 andtest strip800.

Insulation layer

804 is the second layer disposed onsubstrate50.Insulation layer16 includes acutout18 which may have a rectangular shaped structure.Cutout18 exposes a portion of second workingelectrode806, first workingelectrode808, andreference electrode810 which can be wetted with a liquid. The material used forinsulation layer804 and the process for printinginsulation layer804 is the same for bothtest strip62 andtest strip800.

Reagent layer

820 is the third layer disposed onsubstrate50, first workingelectrode808 andreference electrode810. The material used forreagent layer820 and the process for printingreagent layer820 is the same for bothtest strip62 andtest strip800.

Adhesive layer

830 is the fourth layer disposed onsubstrate50. The material used foradhesive layer830 and the process for printingadhesive layer830 is the same for bothtest strip62 andtest strip800. The purpose ofadhesive layer830 is to securetop layer824 totest strip800. In an embodiment of this invention,top layer824 may be in the form of an integrated lance as shown inFIG. 13. In such an embodiment,top layer824 may include alance826 which is located atdistal end58.

Lance

826, 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.Lance826 includes alancet base832 that terminates atdistal end58 of the assembled test strip.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. 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. In addition,lance826 can be fabricated, for example, by a progressive die-stamping technique, as disclosed in the aforementioned International Application No. PCT/GBO1/05634 and U.S. patent application Ser. No. 10/143,399.

FIG. 14 is a simplified schematic showing ameter900 interfacing with a test strip. In an embodiment of this invention the following test strips may be suitable for use withmeter900 which aretest strip62,test strip162,test strip800,test strip2000,test strip3000, ortest strip5000.Meter900 has at least three electrical contacts that form an electrical connection to the second working electrode, the first working electrode, and the reference electrode. In particular second contact (13,103, or812) and reference contact (15,105, or816) connect to afirst voltage source910; first contact (11,101, or814) and the reference contact (15,105, or816) connect to asecond voltage source920.

When performing a test,first voltage source910 applies a first potential E1 between the second working electrode and the reference electrode; andsecond voltage source920 applies a second potential E2 between the first working electrode and the reference electrode. 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 El and second potential E2 may be different. A sample of blood is applied such that the second working electrode, the first working electrode, and the reference electrode are covered with blood. This allows the second working electrode and the first working electrode 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 the second working electrode and the first working electrode.

TABLE 1


Summary of Interference Performance Using Uncorrected and Corrected
Current Output

		Glucose	Inteferent	Normal
		Concen-	Concentration	Concentration
		tration	where effect is	range of
Mode	Interferent	(mg/dL)	signficant	interferent

Un-	Acetaminophen	70	11	1-2
corrected
Un-	Gentisic Acid	70	10	0.05-0.5
corrected
Un-	Uric Acid	70	5	2.6-7.2
corrected
Un-	Acetaminophen	240	16	1-2
corrected
Un-	Gentisic Acid	240	12	0.05-0.5
corrected
Un-	Uric Acid	240	8	2.6-7.2
corrected
Corrected	Acetaminophen	70	120	1-2
Corrected	Gentisic Acid	70	47	0.05-0.5
Corrected	Uric Acid	70	33	2.6-7.2
Corrected	Acetaminophen	240	59	1-2
Corrected	Gentisic Acid	240	178	0.05-0.5
Corrected	Uric Acid	240	29	2.6-7.2

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.