AU2011224097B2 - Systems and methods for discriminating control solution from a physiological sample - Google Patents
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Abstract
- 39 SYSTEMS AND METHODS OF DISCRIMINATING CONTROL SOLUTION FROM A PHYSIOLOGICAL SAMPLE 5 Methods for distinguishing between an aqueous non-blood sample (e.g., a control solution) and a blood sample are provided herein. In one aspect, the method includes using a test strip in which multiple current transients are measured by a meter electrically connected to an electrochemical test strip. The current transients are used to determine if a sample is a blood sample or an aqueous non-blood sample based on 10 at least two characteristics (e.g., amount of interferent present and reaction kinetics). The method can also include calculating a discrimination criteria based upon at least two characteristics. Various aspects of a system for distinguishing between a blood sample and an aqueous non-blood sample are also provided herein. G:TPatent Specifications - Wrd\19469 Specidoc6/09/1 i 0,62 62< 70 di 67 60o 65 yo0 706* FIG. 16
Description
AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION INVENTION TITLE: SYSTEMS AND METHODS OF DISCRIMINTING CONTORL SOLUTION FROM A PHYSIOLOGICAL SAMPLE The following statement is a full description of this invention, including the best method of performing it known to us: 16/09/11 19469 covpag spei I -2 SYSTEMS AND METHODS OF DISCRIMINATING CONTROL SOLUTION FROM A PHYSIOLOGICAL SAMPLE RELATED APPLICATION(S) 5 This application claims priority under 35 U.S.C. §119 to U.S. Provisional Patent Application Serial No. 60/976,083, filed September 28, 2007, entitled "System and Methods of Discriminating Control Solution from a Physiological Sample," the entirety of which is hereby incorporated herein by reference. 10 FIELD The system and method provided herein relates to the field of medical testing, in particular the detection of the presence and/or concentration of an analyte(s) within a sample (e.g., blood). 15 BACKGROUND The present application is a divisional application of Australian Patent Application No. 2008221593, the disclosures of which are herein incorporated by reference. Analyte concentration determination in physiological fluids (e.g., blood or 20 blood derived products such as plasma) is of ever increasing importance in today's society. Such assays find use in a variety of applications and settings, including clinical laboratory testing, home testing, etc., where the results of such testing play a prominent role in the diagnosis and management of a variety of disease conditions. Analytes of interest include glucose for diabetes management, cholesterol for 25 monitoring cardiovascular conditions, and the like. GAPatent Specifications - Word\1 9469 Speci-docl6/09/1 I -3 A common method for analyte concentration determination assays is based on electrochemistry. In such methods, an aqueous liquid sample is placed into a sample reaction chamber in an electrochemical cell made up of at least two electrodes, i.e., a reference and working electrode, where the electrodes have an 5 impedance that renders them suitable for amperometric or coulometric measurement. The component to be analyzed is allowed to react with a reagent to form an oxidizable (or reducible) substance in an amount proportional to the analyte concentration. The quantity of the oxidizable (or reducible) substance present is then estimated electrochemically and related to the analyte concentration in the sample. 10 An automated device, e.g., an electrochemical test meter is typically employed for determining the concentration of the analyte in the sample. Many test meters advantageously allow for an analyte concentration, and usually a plurality of analyte concentrations, to be stored in the memory of the meter. This feature provides the user with the ability to review analyte concentration levels over a period 15 of time, often times as an average of previously collected analyte levels, where such averaging is performed according to an algorithm associated with the meter. However, to ensure that the system is functioning properly, the user will occasionally perform a test using a control fluid instead of a blood sample. Such control fluids (also referred to as control solutions) are generally aqueous solutions having a known 20 concentration of glucose. The user can perform a test with the control solution and compare the displayed results with the known concentration to determine if the system is functioning properly. However, once the control solution test is perfonned, the glucose concentration of the control fluid is stored in the memory of the meter. Thus, when a user seeks to review previous tests and/or the average concentration of 25 previous test results, the results may be skewed to the concentration of the control fluid analyte level. Thus, it is desirable to be able to distinguish control solutions and sample fluids during a test. One option is to manually flag the fluids as either control or test fluids. However automatic flagging would be preferable since it minimizes user 30 interaction and increases ease-of-use. :Patent Speuifications - Word\19469 Speci-doc 16/09/11 -4 As such, there is continued interest in the development of new methods and devices for use in the determination of analyte concentrations in a sample. Of particular interest would be the development of such methods and devices that include the ability to automatically flag a sample as a control fluid or test fluid and to store or 5 exclude measurements accordingly. Of particular interest would be the development of such methods that are suitable for use with electrochemical based analyte concentration determination assays. SUMMARY OF THE INVENTION 10 In one aspect, the present invention provides a method for distinguishing between a blood sample and an aqueous non-blood sample, the method including: (a) introducing a sample into an electrochemical cell, the electrochemical cell including: (i) two electrodes in a spaced apart relationship; and 15 (ii) a reagent; (b) applying a first test potential, having a first polarity, between the electrodes, and measuring cell current; (c) summing at least three current values measured during the first test potential to generate an interferent index; and 20 (d) using the interferent index to distinguish between a blood sample and an aqueous non-blood sample, wherein said summing step is represented by an equation, the equation being isu = i(t), where isum is the summation of current values and t is a time. t=0.05 25 In another aspect, the present invention provides a system for distinguishing between a blood sample and an aqueous non-blood sample, the system including: (a) a test strip including electrical contacts for mating with a test meter and an electrochemical cell including: (i) a first electrode and a second electrode in a spaced apart 30 relationship; and (ii) a reagent; and (b) a test meter including a processor adapted to receive current data from M:\Graham\Clare\Speci & Anndmts\19469 Speci Amdints.Docx, 29/08/12 -5 the test strip, and data storage containing discrimination criteria for distinguishing a blood sample from an aqueous non-blood sample; wherein the discrimination criteria is a first reference value, wherein the test meter distinguishes a blood sample from an aqueous non-blood 5 sample by comparing the first reference value to a pre-determined threshold value, and wherein the first reference value is an interferent index calculated based upon a summation of current values measured during a first current transient, the summation being represented by the equation is. = Li(t), where isum is the summation of current values and t is a time. 10 SUMMARY OF OPTIONAL EMBODIMENTS OF THE INVENTION Various aspects and embodiments of a system and method for distinguishing between an aqueous non-blood sample (e.g., a control solution) and a blood sample are provided herein. In one such embodiment, the methods include using a test strip 15 in which a potential is applied and a current is measured. Current values are used to determine if a sample is a blood sample or a non-blood sample based on at least one characteristic. Further described herein are embodiments of methods for calculating a discrimination criterion based upon at least two characteristics. Still further described herein are embodiments of systems for distinguishing between blood 20 samples and non-blood samples. In one embodiment, a method for distinguishing between a blood sample and a non-blood sample is disclosed. The method includes introducing a sample into an electrochemical cell having first and second electrodes and applying a first test potential between the first electrode and the second electrode. A resulting first 25 current transient is then measured. A second test potential is applied between the first electrode and the second electrode and a second current transient is then measured. The method can also include applying a third test potential between the first electrode and the second electrode, and measuring a third current transient. Based on the first current transient, a first reference value related to the 30 quantity of redox species in the sample is calculated. In addition, based on the second and third current transients, a second reference value related to reaction kinetics is calculated. The first and second reference values are then used to M:\Grahain\Clarc\Speci & Anndmts\19469 Speci Amndmis.Docx, 29/08/12 -6 determine whether the sample is a non-blood sample or a blood sample. The non blood sample can be a control solution or some other sample such as a beverage (e.g., a sports drink such as Gatorade*). In one embodiment, the first reference value is proportional to a 5 concentration of an interferent in the sample. For example, the first reference value can be an interferent index calculated based upon at least one current value from the first current transient. The second reference values can be a function of a percent completion of a chemical reaction. For example, the second reference value can be a residual reaction index calculated based upon at least one current value from the 10 second current transient and at least one current value from the third current transient. In one embodiment, the residual reaction index is calculated based upon a ratio of a second current value and a third current value. In another embodiment, the method can perform the step of measuring a concentration of an analyte in the sample. If the sample is found to be a blood 15 sample, the measured concentration can be stored. Conversely, if the sample is found to be a non-blood sample, the measured concentration can be flagged, stored separately, and/or discarded. In one embodiment, statistical classification can be used to determine if the sample is a non-blood sample or a blood sample. For example, an equation 20 representing an empirically derived discrimination line can be used to evaluate the first and second reference values. In another embodiment, an open-circuit potential is applied to the electrochemical cell before the step of applying the first test potential. In addition, an open-circuit potential can be applied after the step of applying the first test 25 potential. Further described herein is an embodiment of a system for distinguishing between a blood sample and a non-blood sample. In one embodiment, the system can include a test strip and a test meter. The test strip includes electrical contacts for mating with the test meter and an electrochemical cell. The test meter includes a 30 processor configured to receive current data from the test strip, and data storage containing discrimination criteria for distinguishing a blood sample from a non-blood sample based on antioxidant concentration and reaction kinetics. The discrimination M:\Graham\Clare\Speci & Amndmts\19469 Spcci Amndnts.Docx. 29/08/12 -7 criteria can be derived from an interferent index that is representative of antioxidant concentration and a residual reaction index that is representative of reaction kinetics. For example, the discrimination criteria can include an empirically derived discrimination line. The system can further include a non-blood sample (e.g., a 5 control solution) that is substantially devoid of redox species. Still further described herein is an embodiment of a method for calculating a discrimination criterion. The discrimination criterion can be programmed into a test meter for distinguishing between a blood sample and a non-blood sample. In one embodiment, the method includes calculating an interferent index and a residual 10 reaction index for a plurality of non-blood samples and calculating a discrimination criterion based on a regression of the interferent index and the residual reaction index for the plurality of non-blood samples. In one embodiment, the discrimination criterion is a discrimination line. For example, the method can include plotting an interferent index and a residual reaction 15 index for a plurality of blood samples and shifting the discrimination line towards the plurality of blood samples. In one embodiment, a method is provided for distinguishing between a blood sample and a control solution which includes (a) applying a first test potential between a first electrode and a second electrode when a sample is introduced into an 20 electrochemical cell and measuring a first current transient, (b) applying a second test potential between a first electrode and a second electrode wherein the second test potential can be sufficient to oxidize a reduced mediator at the second electrode and measuring a second current transient, (c) applying a third test potential between a first electrode and a second electrode wherein the third test potential can be sufficient 25 to oxidize a reduced mediator at the first electrode. Further, the method can include measuring a third current transient. The method can also include (d) calculating, based on the first current transient, a first reference value, (e) calculating, based on the second and third current transients, a second reference value, and (f) determining, based on the first and second reference values, whether the sample is a control 30 solution or blood sample. The various reference values mentioned above can be determined and/or calculated in various manners. For example, the first reference value can be M:\Graham\Clare\Speci & Amndmts\19469 Speci Amndmts.Docx, 29/08/12 proportional to a concentration of an interferent in the sample, the first reference value can be calculated based upon at least one current value from the first current transient, or the first reference value can be calculated based upon a summation of current values measured during the first current transient. In an embodiment wherein 5 the first reference value can be calculated based upon a summation of current values measured during the first current transient, the summation can be represented by an equation, the equation being i. = Ei(t), where isum is the summation of current t=0.05 values and t is a time. In other embodiments, the second reference value can also be calculated or 10 determined in various manners. For example, the second reference value can be based on a percent completion of a chemical reaction, the second reference value can be based upon at least one current value from the second current transient and at least one current value from the third current transient, or the second reference value can be based upon a second current value at about the end of the second current transient 15 and a third current value at about the beginning of the third current transient. In other embodiments, the second reference value can be based upon a ratio of the second current value and the third current value wherein the ratio can be represented by an equation, the equation being ratio = 2, where i 2 is the second current value 13 and i 3 is the second current value. 20 In various embodiments of the method, various orientations and/or configurations of various components of a system can be utilized. For example, in one embodiment, the first electrode and the second electrode can have an opposing face arrangement wherein a reagent layer can be disposed on the first electrode and not disposed on the second electrode. In another embodiment, the first electrode and 25 the second electrode can have a co-planar arrangement with a reagent layer disposed on the first electrode and not disposed on the second electrode. Various embodiments of the method can also include various additional or optional steps. For example, in one embodiment, the method can include the step of measuring a concentration of an analyte wherein, for example, if the sample is found 30 to be a control solution the analyte concentration associated with the control sample M:\Graham\Clare\Spcci & Amndnts\19469 Speci Amndmts.Docx, 29/08/12 -8A is flagged. Additionally, in one embodiment, above-identified step (f) can further include using a statistical classification to determine if the sample is a control solution or a blood sample. In another embodiment, above-identified step (f) can further include comparing the first reference value to a pre-determined threshold 5 value, and comparing the second reference value to a pre-determined threshold equation (e.g., an equation which is a function of the first reference value) to determine if the sample is a control solution or a blood sample. In another embodiment, a system for distinguishing between a blood sample and a control solution sample is provided. In one embodiment, the system can 10 include (a) a test strip including electrical contacts for mating with a test meter and an electrochemical cell which includes (i) a first electrode and a second electrode in a spaced apart relationship, and (ii) a reagent. Further, the system can include (b) a test meter including a processor configured to receive current data from the test strip, and data storage containing discrimination criteria for distinguishing a blood sample 15 from a control sample (e.g., a sample being substantially devoid of redox species) based on antioxidant concentration and reaction kinetics. In various embodiments, the above-mention discrimination criteria can be derived from various sources. For example, in one embodiment, the discrimination criteria can be derived from an interferent index that is representative of antioxidant 20 concentration and a residual reaction index that is representative of reaction kinetics. In another embodiment, the discrimination criteria can include an empirically derived discrimination line. In yet another embodiment, a method for calculating a discrimination criteria for programming into a test meter for distinguishing between a blood sample and a 25 control solution sample is provided. In one embodiment, the method can include the steps of (a) calculating an interferent index and a residual reaction index for a plurality of control solution samples and (b) calculating a discrimination criteria based on a regression of the interferent index and the residual reaction index for the plurality of control solution samples. Optionally, the method can further include the 30 step of plotting an interferent index and a residual reaction index for a plurality of blood samples and shifting the discrimination line towards the plurality of blood samples. In one embodiment, the discrimination criteria is a discrimination line. M:\Graham\Clare\Speci & Amndmts\i9469 Speci Amndmts.Docx. 29/08/12 -8B In another embodiment, a method for distinguishing between a blood sample and a control solution sample is provided which includes the step of (a) introducing a sample into an electrochemical cell wherein the cell can include (i) two 5 electrodes in a spaced apart relationship and (ii) a reagent. The method can further include the M:\Graham\Clare\Speci & Amndmts\19469 Speci Amndmts.Docx, 29/08/12 -9 steps of (b) applying a first test potential, having a first polarity, between the electrodes, and measuring cell current, (c) summing at least two current values measured during the first test potential to generate an interferent index, and (d) using the interferent index to distinguish between a blood sample and a control solution 5 sample. BRIEF DESCRIPTION OF THE DRAWINGS Various features of the present disclosure are set forth with particularity in the appended claims. A better understanding of such features can be obtained by 10 reference to the following detailed description that sets forth illustrative, non-limiting embodiments and the accompanying drawings of which: FIG. lA is a perspective view of an exemplary assembled test strip; 15 FIG. lB is an exploded perspective view of the test strip of FIG. lA; FIG. 1 C is an expanded perspective view of a proximal portion of the test strip of FIG. IA; FIG. 2 is a bottom plan view of the test strip of FIG. 1A; 20 FIG. 3 is a side plan view of the test strip of FIG. lA; FIG. 4A is a top plan view of the test strip of FIG. lA; 25 FIG. 4B is an expanded partial side view of the proximal portion of the test strip consistent with arrows 4A-4A of FIG. 4A; FIG. 5 is a simplified schematic showing a test meter electrically interfacing with portions of the test strip; 30 FIG. 6 shows an example of a potential waveform in which the test meter applies a series of open-circuit potentials and test potentials for prescribed time intervals; 35 FIG. 7 shows a current transient generated by the test meter that is testing the test strip with the potential waveform of FIG. 6 with a control solution sample (CS, G 'Patent Specifications - Word\19469 Specidoc 16/I9 1 -10 dotted line) and a blood sample (BL, solid line); FIG. 8 shows the summation of current values at 0.2 and 0.5 seconds for a control solution, plasma, a blood sample with 48% hematocrit, and a blood sample 5 with 77% hematocrit when a potential of 20 mV was applied; FIG. 9 is an expanded view of FIG. 7 showing a first test current transient and second test current transient for control solution (CS) and blood (BL); 10 FIG. 10 is a chart showing a non-linear relationship between the percent of substrate consumed and the residual reaction index for blood samples having various hematocrit levels and for control solution (diamonds =25% hematocrit blood, squares = 42% blood, triangles = 60% hematocrit blood, x = control solution); 15 FIG. 11 is a chart showing a relationship between an interferent index and a residual reaction index for a plurality of blood samples (diamonds) and control solution samples (squares); FIG. 12 shows an example of another embodiment of a test potential 20 waveform in which the test meter applies a plurality of test voltages for prescribed time intervals; FIG. 13 shows a test current transient generated with the test voltage waveform of FIG. 12; 25 FIG. 14 is a chart showing a relationship between an interferent index and a residual reaction index using an alternative algorithm tested at a wide range of temperatures, hematocrit levels, and glucose concentrations; and FIG. 15 is a chart showing a relationship between an interferent index and a 30 residual reaction index using the alternative algorithm tested only at ambient temperature. DETAILED DESCRIPTION Certain exemplary embodiments will now be described to provide an overall 35 understanding of the principles of the structure, function, manufacture, and use of the G:Patent Specificaons - Word\l9469 SpecLdoc 6/09/11 - 11 systems and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the systems and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the 5 scope of the present disclosure is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present disclosure. The presently disclosed systems and methods are suitable for use in the 10 determination of a wide variety of analytes in a wide variety of samples, and are particularly suited for use in the determination of analytes in whole blood or derivatives thereof, where an analyte of particular interest is glucose. In one aspect, the present disclosure provides various embodiments of a method for determining whether a sample applied to a test strip is an aqueous non-blood sample (e.g., a 15 control solution) or a blood sample. In one such embodiment, at least two characteristics are used to distinguish between a blood sample and a non-blood sample. This description will focus on distinguishing between blood samples and control solutions. However, as shown in Example 2 below, the systems and methods provided herein are equally applicable to distinguishing blood samples from any of a 20 variety of non-blood samples (e.g., beverages including sports drinks such as Gatorade*). The methods provided herein may be used, in principle, with any type of electrochemical cell having spaced apart first and second electrodes and a reagent layer. For example, an electrochemical cell can be in the form of a test strip. In one 25 aspect, the test strip includes two opposing electrodes separated by a thin spacer layer, where these components define a sample reaction chamber or zone in which is located a reagent layer. One skilled in the art will appreciate that other types of test strips, including, for example, test strips with co-planar electrodes could also be used with the methods described herein. 30 FIGS. IA-4B show various views of an exemplary test strip 62 suitable for use with the methods described herein. As shown, the test strip 62 can include an elongate body extending from a proximal end 80 to a distal end 82, and having Q:\Patent Specifications - Wordi19469 Specidoc 16109111 - 12 lateral edges 56, 58. The proximal portion of the body 59 can include a sample reaction chamber 61 having multiple electrodes 164, 166 and a reagent 72, while the distal portion of the test strip body 59 can include features configured for electrically communicating with a test meter. In use, physiological fluid or a control solution 5 can be delivered to the sample reaction chamber 61 for electrochemical analysis. In the illustrative embodiment, the test strip 62 can include a first electrode layer 66 and a second electrode layer 64, with a spacer layer 60 positioned therebetween. The first electrode layer 66 can provide a first electrode 166 and a first connection track 76 for electrically connecting the first electrode 166 to a first 10 electrical contact 67. Similarly, the second electrode layer 64 can provide a second electrode 164 and a second connection track 78 for electrically connecting the second electrode 164 with a second electrical contact 63. In one embodiment, the sample reaction chamber 61 is defined by the first electrode 166, the second electrode 164, and a spacer 60 as shown in FIGS. I A-4B. 15 Specifically, the first electrode 166 and the second electrode 164 define, respectively, the bottom and top of the sample reaction chamber 61. A cutout area 68 of the spacer 60 can define the side walls of the sample reaction chamber 61. In one aspect, the sample reaction chamber 61 can further include an number of ports 70 that provide a sample inlet and/or a vent. For example, one of the ports can provide a 20 fluid sample ingress and the other port can act as a vent. The sample reaction chamber 61 can have a small volume. For example, the volume can range from about 0.1 microliters to about 5 microliters, preferably about 0.2 microliters to about 3 microliters, and more preferably about 0.3 microliters to about 1 microliter. As will be appreciated by those skilled in the art, the sample 25 reaction chamber 61 can have various other such volumes. To provide the small sample volume, the cutout 68 can have an area ranging from about 0.01 cm 2 to about 0.2 cm 2 , preferably about 0.02 cm 2 to about 0.15 cm 2 , and more preferably about 0.03 cm 2 to about 0.08 Cm 2 . Similarly, those skilled in the art will appreciate that the volume cutout 68 can be of various other such areas. In addition, the first and second 30 electrode 166, 164 can be spaced in the range of about 1 micron to about 500 microns, preferably between about 10 microns and about 400 microns, and more preferably between about 40 microns and about 200 microns. In other embodiments, GAPatent Specitications - Womd\19469 Speci.docl6/09/] I - 13 such a range can vary between various other values. The close spacing of the electrodes can also allow redox cycling to occur, where oxidized mediator generated at the first electrode 166, can diffuse to the second electrode 164 to become reduced, and subsequently diffuse back to the first electrode 166 to become oxidized again. 5 At the distal end of the test strip body 59, a first electrical contact 67 can be used to establish an electrical connection to a test meter. A second electrical contact 63 can be accessed by the test meter through a U-shaped notch 65 as illustrated in FIG. 2. One skilled in the art will appreciate that the test strip 62 can include a variety of alternative electrical contact configured for electrically connecting to a test 10 meter. For example, U.S. Patent No. 6,379,513, the entirety of which is hereby incorporated herein by reference, discloses an electrochemical cell connection means. [0001] In one embodiment, the first electrode layer 66 and/or the second electrode layer 64 can be a conductive material formed from materials such as gold, palladium, 15 carbon, silver, platinum, tin oxide, iridium, indium, and combinations thereof (e.g., indium doped tin oxide). In addition, the electrodes can be formed by disposing a conductive material onto an insulating sheet (not shown) by various process such as, for example, a sputtering, electroless plating, or a screen printing process. In one exemplary embodiment, the second electrode layer 64 can be a sputtered gold 20 electrode and the first electrode layer 66 can be a sputtered palladium electrode. Suitable materials that can be employed as the spacing layer 60 include various insulating materials, such as, for example, plastics (e.g., PET, PETG, polyimide, polycarbonate, polystyrene), silicon, ceramic, glass, adhesives, and combinations thereof. 25 A reagent layer 72 can be disposed within the sample reaction chamber 61 using a process such as slot coating, dispensing from the end of a tube, ink jetting, and screen printing. Such processes are described, for example, in the following U.S. Patent Nos. 6,749,887; 6,869,411; 6,676,995; and 6,830,934, the entirety of each of these references being incorporated herein by reference. In one embodiment, 30 the reagent layer 72 can include at least a mediator and an enzyme, and can be deposited onto the first electrode 166. Various mediators and/or enzymes are within the spirit and scope of the present disclosure. For example, suitable mediators G:\Patent Specifications - Word\19469 Speci.doc1609il 1 -14 include ferricyanide, ferrocene, ferrocene derivatives, osmium bipyridyl complexes, and quinone derivatives. Examples of suitable enzymes include glucose oxidase, glucose dehydrogenase (GDH) based on pyrroloquinoline quinone (PQQ) co-factor, GDH based on nicotinamide adenine dinucleotide co-factor, and FAD-based GDH 5 [E.C. 1.1.99.10]. One exemplary reagent formulation, which would be suitable for making the reagent layer 72, is described in pending U.S. Application No. 10/242,951, entitled, "Method of Manufacturing a Sterilized and Calibrated Biosensor-Based Medical Device", published as U.S. Published Patent Application No. 2004/0120848, the entirety of which is hereby incorporated herein by reference. 10 Either the first electrode 166 or the second electrode 164 can function as working electrode which oxidizes or reduces a limiting amount of mediator depending on the polarity of the applied test potential of the test meter. For example, if the current limiting species is a reduced mediator, it can be oxidized at the first electrode 166 as long as a sufficiently positive potential was applied with respect to 15 the second electrode 164. In such a situation, the first electrode 166 performs the function of the working electrode and second electrode 164 performs the function of a counter/reference electrode. It should be noted that unless otherwise stated for test strip 62, all potentials applied by test meter 100 will hereinafter be stated with respect to second electrode 164. 20 Similarly, if a sufficiently negative potential is applied with respect to the second electrode 164, then the reduced mediator can be oxidized at the second electrode 164. In such a situation, the second electrode 164 can perform the function of the working electrode and the first electrode 166 can perform the function of the counter/reference electrode. 25 A first step in an embodiment of the presently disclosed method can include introducing a quantity of the fluid sample of interest into the test strip 62, which includes the first electrode 166, the second electrode 164 and a reagent layer 72. The fluid sample can be whole blood or a derivative or fraction thereof, or a control solution. The fluid sample, e.g., blood, can be dosed into the sample reaction 30 chamber 61 via the port 70. In one aspect, the port 70 and /or the sample reaction chamber 61 can be configured such that capillary action causes the fluid sample to fill the sample reaction chamber 61. G Patent Speifications - Word\ 19469 Spectdoc 6/09/11 - 15 FIG. 5 provides a simplified schematic of a test meter 100 interfacing with a first electrical contact 67 and a second electrical contact 63, which are in electrical communication with the first electrode 166 and the second electrode 164, respectively, of the test strip 62. The test meter 100 can be configured to electrically 5 connect to the first electrode 166 and the second electrode 164 via a first electrical contact 67 and a second electrical contact 63, respectively (as shown in FIGS. 2 and 5). As will be appreciated by those skilled in the art, a variety of test meters can be used with the method described herein. However, in one embodiment, the test meter includes at least a processor configured for performing calculations capable of 10 discriminating between blood and a control sample, as well as configured for data sorting and/or storage. As illustrated in FIG. 5, an electrical contact 67 can include two prongs 67a, 67b. In one exemplary embodiment, the test meter 100 separately connects to the prongs 67a, 67b, such that when the test meter 100 interfaces with a test strip 62 a 15 circuit is completed. The test meter 100 can measure the resistance or electrical continuity between the prongs 67a, 67b to determine whether the test strip 62 is electrically connected to the test meter 100. One skilled in the art will appreciate that the test meter 100 can use a variety of sensors and circuits to determine when the test strip 62 is properly positioned with respect to the test meter 100. 20 in one embodiment, test meter 100 can apply a test potential and/or a current between first electrical contact 67 and second electrical contact 63. Once test meter 100 recognizes that strip 62 has been inserted, test meter 100 turns on and initiates a fluid detection mode. In one embodiment, the fluid detection mode causes test meter 100 to apply a constant current of I microampere between first electrode 166 and 25 second electrode 164. Because test strip 62 is initially dry, test meter 100 measures a maximum voltage, which is limited by the hardware within test meter 100. However, once a user doses a fluid sample onto inlet 70, this causes sample reaction chamber 61 to become filled. When the fluid sample bridges the gap between first electrode 166 and second electrode 164, test meter 100 will measure a decrease in 30 measured voltage (e.g., as described in U.S. Patent No. 6,193,873, the entirety of which being incorporated herein by reference), which is below a predetermined threshold causing test meter 100 to automatically initiate the glucose test. G:Patent Specifications - Word\19469 Speui.doc 16/09/LI - 16 It should be noted that the measured voltage may decrease below a pre determined threshold when only a fraction of the sample reaction chamber 61 has been filled. A method of automatically recognizing that a fluid was applied does not necessarily indicate that the sample reaction chamber 61 has been completely filled, 5 but can only confirm a presence of some amount of fluid in the sample reaction chamber 61. Once the test meter 100 determines that a fluid has been applied to test strip 62, a short, but non-zero amount of time may still be required to allow the fluid to completely fill the sample reaction chamber 61. In one embodiment, once the test meter 100 has determined that a fluid has 10 been introduced (e.g., dosed) onto the test strip 62, a test meter 100 can perform a glucose test by applying a plurality of open-circuit potentials and a plurality of test potentials to the test strip 62 for prescribed intervals as shown in FIG. 6. A glucose test time interval To, represents an amount of time to perform the glucose test (but not necessarily all the calculations associated with the glucose test) where the glucose 15 test time interval TG can include a first open-circuit time interval TocI, a first test potential time interval TI, a second open-circuit time interval Toc2, a second test potential time interval T 2 , and a third test potential time interval T 3 . The glucose test time interval TG can range, for example, from about 1 second to about 5 seconds. While the two open-circuit time intervals and the three test potential time intervals 20 are described, one skilled in the art will appreciate that the glucose test time interval can include different numbers of open-circuit and test potential time intervals. For example, the glucose test time interval could include a single open-circuit time interval and/or only two test potential time intervals. Once the glucose assay has been initiated, the test meter 100 switches to a 25 first open-circuit for a first open-circuit potential time interval Toci, which in the illustrated embodiment is about 0.2 seconds. In another embodiment, the first open circuit time interval Toci can be in the range of about 0.05 seconds to about 2 seconds and preferably between about 0.1 seconds to about 1.0 seconds, and most preferably between about 0.15 seconds to about 0.6 seconds. 30 One of the reasons for implementing the first open-circuit is to allow sufficient time for the sample reaction chamber 61 to fill or partially fill with sample. Typically, at ambient temperature (i.e., 22 C), the sample reaction chamber 61 takes G:APatent Speficaions - WordU9469 Sper.daco16/09/11 - 17 about 0.1 seconds to about 0.5 seconds to completely fill with blood. Conversely, at ambient temperature, the sample reaction chamber 61 takes about 0.2 seconds or less to completely fill with control solution, where the control solution is formulated to have a viscosity of about I to about 3 centipoise. 5 While the control solutions are composed of known components and are generally uniform, blood samples can vary in their make-up and/or composition. For example, high hematocrit blood samples are more viscous than low hematocrit blood samples, therefore higher hematocrit blood samples require additional time to fill compared with lower hematocrit blood samples. Thus, depending on a variety of 10 factors, blood sample filling time can vary. After applying the first open-circuit potential, the test meter 100 applies a first test potential E 1 between the first electrode 166 and the second electrode 164 (e.g., -0.3 Volts in FIG. 6), for a first test potential time interval T 1 (e.g., 0.15 seconds in FIG. 6). The test meter 100 measures the resulting first current transient, 15 which can be referred to as ia(t) as shown in FIG. 7. A current transient represents a plurality of current values measured by a test meter during a particular test potential time interval. In one embodiment, the first test potential time interval T, can be in the range of about 0.05 seconds to about 1.0 second and preferably between about 0.1 seconds to about 0.5 seconds, and most preferably between about 0.1 seconds to 20 about 0.2 seconds. In other embodiments, the first test potential time interval T 1 can include any other desired time ranges. As discussed below, a portion or all of the first current transient can be used in the methods described herein to determine whether a control solution or a blood sample was applied to the test strip 62. The magnitude of the first transient current is 25 effected by the presence of easily oxidizable substances in the sample. Blood usually contains endogenous and exogenous compounds that are easily oxidized at second electrode 164. Conversely, the control solution can be formulated such that it does not contain oxidizable compounds. However, blood sample composition can vary and the magnitude of the first current transient for high viscosity blood samples will 30 typically be smaller than low viscosity samples (in some cases even less than the control solution samples) because the sample reaction chamber 61 may be not be completely filled after about 0.2 seconds. An incomplete fill will cause the effective G:Patent Specifications - Word\19469 Specidoc16/09/1 I - 18 area of the first electrode 166 and the second electrode 164 to decrease which in turn can cause the first current transient to decrease. Thus, the presence of oxidizable substances in a sample, by itself, is not always a sufficient discriminatory factor because of variations in blood samples. 5 After the test meter 100 stops applying the first test potential El, it can be configured to switch to a second open-circuit for a second open-circuit time interval Toc2, which in the example of FIG. 6, is about 0.65 seconds. In another embodiment, a second open-circuit time interval Toc2 can be in the range of about 0.1 seconds to about 2.0 seconds and preferably between about 0.3 seconds to about 10 1.5 seconds, and most preferably between about 0.5 seconds to about 1.0 seconds. In other embodiments, the second open-circuit time interval Toc2 can be any other time interval as desired. One of the reasons for implementing the second open-circuit is to provide sufficient time for the sample reaction chamber 61 to completely fill, to allow the 15 reagent layer 72 to dissolve, and to allow a reduced mediator and an oxidized mediator to re-equilibrate at the respective first electrode 166 and second electrode 164 from the perturbation caused by the first test potential EI. Although the sample reaction chamber 61 typically fills rapidly, the second open-circuit time interval Toc 2 can be sufficiently long to account for conditions which can cause the fill times to 20 increase such as low ambient temperature (e.g., about 5 "C) and high hematocrit levels (e.g., >60% bematocrit). During the first test potential El, a reduced mediator can be depleted at the second electrode 164 and can be generated at the first electrode 166 to form a concentration gradient. The second open-circuit potential provides time for the 25 reduced mediator concentration profile to become closer to the state immediately before first test potential E, was applied. As will be described below, a sufficiently long second open-circuit potential is useful because it can allow for glucose concentration to be calculated in the presence of interferents. An alternative embodiment test potential EI' can be applied between the 30 electrodes for a duration between when the meter can detect that the strip is filling with sample and before a second test potential E2 is applied. In one aspect, the test potential E i' is small. For example, the potential can be between about -I to about G:Palent Specifications - Word\19469 Speci.docl6/09/11 - 19 100 mV, preferably between about -5 mV and about -50 mV and most preferably between about -10 mV and about -30 mV. The smaller potential perturbs the reduced mediator concentration gradient to a lesser extent compared to applying a larger potential difference, but is still sufficient to obtain a measure of the oxidizable 5 substances in the sample. The test potential E' can be applied for a portion of the time between detection of fill and when the second test potential E 2 is applied or can be applied for the whole of that time period. If the test potential E is to be used for a portion of the time then an open-circuit could be applied for the remaining portion of the time. The combination of any number of open-circuit and small voltage 10 potential applications, their order and times applied is not critical in this embodiment, can be applied as long as the total period for which the small potential EI' is applied is sufficient to obtain a current measurement indicative of the presence and/or quantity of oxidizable substances present in the sample. In a preferred embodiment, the small potential E 1 is applied for substantially the entire period 15 between when a fill is detected and when the second test potential E 2 is applied. Once the second open-circuit time interval Toc2 or an equivalent time in the small potential E 1 embodiment has elapsed, the test meter 100 can apply a second test potential E 2 between the first electrode 166 and the second electrode 164 for a second test potential time interval T 2 . During the second test potential time interval 20 T 2 , the test meter 100 can measure a second current transient ih(t). After the second potential time interval T 2 has elapsed, the test meter 100 can apply a third test potential E 3 between the first electrode 166 and the second electrode 164 for a third test potential time interval T 3 , which may be referred to as ic(t). The second test potential time interval T 2 and the third test potential time interval T3 can each range 25 from about 0.1 seconds to about 4 seconds. For the embodiment shown in FIG. 6, the second test potential time interval T2 was about 3 seconds and the third test potential time interval T 3 was about 1 second. As mentioned above, an open circuit potential time period can be allowed to elapse between the second test potential E2 and the third test potential E 3 . Alternatively, the third test potential E3 can be applied 30 following the application of the second test potential E 2 . Note that a portion of the first, second, or third current transient may be generally referred to as a cell current or a current value. G:\Patent Specifications - Word\19469 Speci.doc16/09/U1 -20 In one embodiment, the first test potential E and the second test potential E 2 both have a first polarity, and the third test potential E 3 has a second polarity, which is opposite to the first polarity. However, one skilled in the art will appreciate that the polarity of the first, second, and third test potentials can be chosen depending on 5 the manner in which analyte concentration is determined and/or depending on the manner in which the test samples and control solutions are distinguished. The first test potential E, and the second test potential E2 can be sufficiently negative in magnitude with respect to the second electrode 164 such that the second electrode 164 functions as a working electrode in which a limiting oxidation current 10 is measured. Conversely, the third test potential E 3 can be sufficiently positive in magnitude with respect to the second electrode 164 such that the first electrode 166 functions as a working electrode in which a limiting oxidation current is measured. A limiting oxidation occurs when all the oxidizable species have been locally depleted at the working electrode surface such that the measured oxidation current is 15 proportional to the flux of oxidizable species diffusing from the bulk solution towards the working electrode surface. The term bulk solution refers to a portion of the solution sufficiently far away from the working electrode where the oxidizable species was not located within the depletion zone. The first test potential Ej, the second test potential E 2 , and the third test potential E 3 can range from about -0.6 20 Volts to about +0.6 Volts (with respect to the second electrode 164) when using either a sputtered gold or palladium working electrode and a ferricyanide mediator. FIG. 7 shows first, second, and third current transients generated by a test meter 100 and a test strip 62 using either a control solution sample (dotted line) or a blood sample (solid line). The control solution sample contained a 525 mg/dL 25 glucose concentration and the blood sample contained a 530 mg/dL glucose concentration with about 25% hematocrit. FIG. 8 shows an expanded view of first and second current transients in FIG. 7. FIGS. 7 and 8 show the resulting current transients when applying the potential waveform shown in FIG. 6. The description below details how the current transients can be converted into an accurate glucose 30 measurement for the test solution or control solution. As illustrated in FIG. 12, the test meter 100 can perform a glucose test by applying a plurality of test potentials for prescribed intervals. The plurality of test G:\Patent Specificaboas -Word\] 9469 Specidoc16/09/]l -21 potentials may include a first test potential EI' for a first test potential time interval
T
1 , a second test potential E2 for a second test potential time interval T 2 , and a third test potential E3 for a third test potential time interval T 3 . The plurality of test current values measured during the first, second, and third test potential time intervals may 5 be performed at a frequency ranging from about I measurement per nanosecond to about one measurement per 100 milliseconds. One skilled in the art will appreciate that names "first," "second," and "third" are chosen for convenience and do not necessarily reflect the order in which the test potentials are applied. Once the glucose assay has been initiated using the test potential waveform of 10 FIG. 12, the test meter 100 may apply a first test potential El' (e.g., -20 mV) for a first test potential time interval Ti (e.g., between about 0 and about I second). The first test potential time interval TI can range from about 0.1 seconds to about 3 seconds and preferably range from about 0.2 seconds to about 2 seconds, and most preferably range from about 0.3 seconds to about 1 seconds. The first test potential 15 time interval T, may be sufficiently long so that the sample reaction chamber 61 can fully fill with sample and also so that the reagent layer 72 can at least partially dissolve or solvate. In one aspect, the first test potential E 1 ' may be a value relatively close to the redox potential of the mediator so that a relatively small amount of a reduction or oxidation current is measured. FIG. 13 shows that a relatively small 20 amount of current may be observed during the first test potential time interval compared to during the second and third test potential time intervals. For example, when using ferricyanide and/or ferrocyanide as the mediator, the first test potential E' can range from about -100 mV to about -I mV, preferably range from about -50 mV to about -5 mV, and most preferably range from about -30 mV to about -10 mV. 25 After applying the first test potential El', the test meter 100 can apply a second test potential E2 between the first electrode 166 and the second electrode 164 (e.g., about -0.3 Volts as illustrated in FIG. 12), for a second test potential time interval T 2 (e.g., about 3 seconds as illustrated in FIG. 12). The second test potential
E
2 may be a value sufficiently negative of the mediator redox potential so that a 30 limiting oxidation current occurs at the second electrode 164. For example, when using ferricyanide and/or ferrocyanide as the mediator, the second test potential E2 can range from about -600 mV to about zero mV, preferably range from about -600 G:Patent Specifications - Word\19469 Specudoc16/09/1 l - 22 mV to about -100 mV, and more preferably be about -300 mV. The second test potential time interval T 2 may be sufficiently long to monitor the rate of generation of reduced mediator (e.g., ferrocyanide) in the sample reaction chamber 61 based on the magnitude of a limiting oxidation current. The reduced 5 mediator may be generated by a series of chemical reactions in the reagent layer 72. During the second test potential time interval T 2 , a limiting amount of reduced mediator is oxidized at the second electrode 164 and a non-limiting amount of oxidized mediator is reduced at the first electrode 166 to form a concentration gradient between the first electrode 166 and the second electrode 164. As will be 10 described, the second test potential time interval T 2 should be sufficiently long so that a sufficient amount of ferricyanide can be generated at the second electrode 164. A sufficient amount of ferricyanide may be required at the second electrode 164 so that a limiting current can be measured for oxidizing ferrocyanide at the first electrode 166 during the third test potential E 3 . The second test potential time 15 interval T 2 can range from about 0 seconds to about 60 seconds and preferably range from about I second to about 10 seconds, and most preferably range from about 2 seconds to about 5 seconds. FIG. 13 shows a relatively small peak at the beginning of the second test potential time interval T2 followed by a gradual increase of an absolute value of an 20 oxidation current during the second test potential time interval (e.g., between about I second to about 4 seconds). The small peak occurs due to an initial depletion of reduced mediator at about I second. The gradual increase in oxidation current is ascribed to the generation of ferrocyanide by reagent layer 72 followed by its diffusion to the second electrode 164. 25 After applying the second test potential E2, the test meter 100 can apply a third test potential E 3 between the first electrode 166 and the second electrode 164 (e.g., +0.3 Volts in FIG. 12), for a third test potential time interval T3 (e.g., between about 4 to about 5 seconds as illustrated in FIG. 12). The third test potential E 3 may be a value sufficiently positive of the mediator redox potential so that a limiting 30 oxidation current is measured at the first electrode 166. For example, when using ferricyanide and/or ferrocyanide as the mediator, the third test potential E3 can range from about zero mV to about 600 mV, preferably range from about 100 mV to about G:\raenI Specifications - Word\]9469 Specidoc16/0911 I - 23 600 mV, and more preferably be about 300 mV. The third test potential time interval T 3 may be sufficiently long to monitor the diffusion of a reduced mediator (e.g., ferrocyanide) near the first electrode 166 based on the magnitude of the oxidation current. During the third test potential time 5 interval T 3 , a limiting amount of reduced mediator is oxidized at the first electrode 166 and a non-limiting amount of oxidized mediator is reduced at the second electrode 164. The third test potential time interval T 3 can range from about 0.1 seconds to about 5 seconds and preferably range from about 0.3 seconds to about 3 seconds, and most preferably range from about 0.5 seconds to about 2 seconds. 10 FIG. 13 shows a relatively large peak at the beginning of the third test potential time interval T 3 followed by a decrease to a steady-state current. In one embodiment, the second test potential E 2 may have a first polarity and third test potential E 3 may have a second polarity that is opposite to the first polarity. However, one skilled in the art will appreciate that the polarity of the second and 15 third test potentials can be chosen depending on the manner in which analyte concentration is determined. Assuming that a test strip has an opposing face or facing arrangement as shown in FIGS. IA-4B, and that a potential waveform is applied to the test strip as shown in FIG. 6, a glucose concentration can be calculated using a glucose algorithm 20 as shown in Equation (Eq.) 1: Eq. I [G]=(_) x(axi, -Z) In Eq. 1, [G] is the glucose concentration, ii is a first current value, i2 is a 25 second current value, and is is a third current value, and the termsp, Z, and a are empirically derived calibration constants. A derivation of Eq. I can be found in a pending U.S. Published Patent Application No. 2007/0074977 (U.S. Application Ser. No. 11/240,797), filed on September 30, 2005 and entitled "Method and Apparatus for Rapid Electrochemical Analysis," the entirety of which is hereby incorporated 30 herein by reference. The first current value it and the second current value i2 are calculated from the third current transient and the third current value i3 is calculated G:Patent Specifications - Word\19469 Speci.doc16/09/11 - 24 from the second current transient. One skilled in the art will appreciate that the names "first," "second," and "third" are chosen for convenience and do not necessarily reflect the order in which the current values are calculated. In addition, all current values (e.g., it, i2, and i3) stated in Eq. 1 use the absolute value of the 5 current. In another embodiment, the term i 1 can be defined to include peak current values from the second and third current transients to allow for more accurate glucose concentrations in the presence of interferents as shown in Eq. 2: q i, -- 2i +i, 10 Eq. 2 7 p= x The term ipb represents a peak current value for the second test potential time interval T2 and the term i, represents a peak current value for the third test potential time interval T 3 . The term i, is an estimate of the steady-state current, which is the 15 current predicted to occur at long times after the application of the third test potential
E
3 in the absence of on-going chemical reactions. Where Eq. 2 is used, the second open-circuit potential time interval Toc2 is preferably sufficiently long so as to allow Eq. 2 to compensate for the presence of interferents. When the second open-circuit potential time interval Toc2 is too short, the second peak current value iph can become 20 distorted and can reduce the effectiveness of the interferent correction calculations. The use of peak current values to account for interferents in a physiological sample are described in U.S. Published Patent Application No. 2007/0227912 (U.S. Patent Application Serial No. 11/278,341), filed on March 31, 2006 and entitled "Methods and Apparatus for Analyzing a Sample in the Presence of Interferents," the entirety 25 of which is hereby incorporated herein by reference. In one embodiment, Eq. 1 and Eq. 2 can be used together to calculate a glucose concentration for either blood or a control solution. In another embodiment, the algorithm of Eq. 1 and Eq. 2 can be used for blood with a first set of calibration factors (i.e. a, p, and Z) and a second set of calibration factors can be used for the 30 control solution. When using two different sets of calibration factors, the methods described herein for discriminating between a test fluid and a control solution can OPatent Specifications - wordI 9469 Speci.douv6i9/11 - 25 improve the effectiveness of the analyte concentration calculations. In addition, if the test meter determines that the sample is control solution (as opposed to blood), the test meter can store the resulting glucose concentration of the control sample such that a user can review test sample concentration data separately 5 from control solution data. For example, the glucose concentrations for control solutions can be stored in a separate database, can be flagged, and/or discarded (i.e., not stored or stored for a short period of time). Another advantage of being able to recognize a control solution is that a test meter can be programmed to automatically compare the results (e.g., glucose 10 concentration) of the test of the control solution with the expected glucose concentration of the control solution. For example, the test meter can be pre programmed with the expected glucose level(s) for the control solution(s). Alternatively, a user could input the expected glucose concentration for the control solution. When the test meter recognizes a control solution, the test meter can 15 compare the measured control solution glucose concentration with the expected glucose concentration to determine if the meter is functioning properly. If the measured glucose concentration is out of the expected range, the test meter can output a warning message to alert the user. In one embodiment, the method described herein uses the presence of redox 20 species to distinguish a control solution from a blood sample. The method can include the step of applying a first test potential E1' and using one or more current values measured during the test potential as a discriminator. In one aspect, two current values from the first test potential E ' are summed and used as the discriminator. FIG. 8 shows data for a control solution, plasma, a blood sample with 25 48% hematocrit, and a blood sample with 77% hematocrit. A potential of about 20 mV was applied for the first 1 second and current values at about 0.2 to about 0.5 seconds were summed. As shown in FIG. 8, the summed current values were sufficient to distinguish between a control solution (that was substantially devoid of interferents) and blood samples. 30 In another embodiment, two characteristics of control solution are used to distinguish control solutions from blood - the presence and/or concentration of redox species in the sample and reaction kinetics. The method disclosed herein can include G:\Patent Specifications - Word\ 19469 SpeeldocI6/09/l I -26 the step of calculating a first reference value that is representative of the redox concentration in the sample and a second reference value that is representative of the rate of reaction of the sample with the reagent. In one embodiment, the first reference value is an interferent oxidation current and the second reference value is a 5 reaction completion percentage. In regard to redox species in the sample, blood usually contains various endogenous redox species or "interferents" such as ascorbic acid and uric acid, as well as exogenously derived interferents such as gentisic acid (gentisic acid is a metabolite of aspirin). Endogenous interferents are chemical species that can be 10 easily oxidized at an electrode and are usually present in blood within a physiological range for healthy individuals. Exogenously derived interferents are also a chemical species that can be easily oxidized at an electrode, but are not usually present in blood unless they are inputted into the body via consumption, injection, absorption, and the like. 15 A control solution can be formulated to be either essentially free of antioxidants or to have a relatively high interferent concentration compared to the interferent concentration in a blood sample. For the case in which the control solution is essentially free of antioxidants, the magnitude of the first current transient should be smaller for the control solution than for a blood sample as shown in FIG. 20 9. For the case in which the control solution has a relatively high concentration of interferents, the magnitude of the first current transient should be larger for the control solution than for a blood sample (data not shown), A first reference value can be calculated based on the current values within the first current transient. In one embodiment, the first reference value can include a 25 summation of current values at two points in time during the first current transient. In one example, the current values at about 0.3 and about 0.35 seconds can be used when employing the test potential waveform of FIG. 6. In another embodiment when a test potential Ei is applied for the entire period between when fill is detected and the second test potential E 2 , the first reference value is preferably obtained by 30 summing two values over a longer period, for example about 0.2 seconds to about 0.5 seconds. In yet another embodiment, the first reference value can be obtained by a summation of the current values obtained during the first time current transient G:\Patnt Specificaions - Word\19469 Speci.doc16/09/11 -27 when using the test potential waveform of FIG. 12. As an example, the summation can be represented by Eq. 3: Eq. 3 i = Zi(t) 1=0_05 5 The terms irn is the summation of current values and t is a time. The first reference value can be referred to as an interferent index because it is proportional to the interferent concentration and should not substantially depend on the glucose concentration. Therefore, in theory, the test meter should be able to distinguish whether the sample is blood or control solution based on the interferent 10 index. However, in practice, using only the interferent index did not always sufficiently discriminate between blood and the control solution. Although blood typically has a much higher interferent concentration, there are certain conditions in which the first current transient for blood may be attenuated such that it is comparable to control solution. These conditions include high glucose 15 concentration, high bematocrit, low temperature, and incomplete filling of the sample reaction chamber 61. Thus, in one embodiment, an additional factor was implemented to enable the test meter to sufficiently discriminate between blood and control solution. The additional factor used for helping discriminate between blood and control 20 solution can be a second reference value. The second reference value may be referred to as a residual reaction index, which can be a value which is a function of the percent of remaining substrate which would have reacted if given enough time. The residual reaction index relates to the reaction rate in that a high reaction rate can cause the substrate to be depleted by the reaction. However, the residual reaction 25 index will also depend on the initial magnitude of the substrate concentration. The reagent layer 72 can include glucose dehydrogenase (GDH) based on the PQQ co-factor and ferricyanide. In another embodiment, the enzyme GDH based on the PQQ co-factor may be replaced with the enzyme GDH based on the FAD co factor. When blood or control solution is dosed into a sample reaction chamber 61, 30 glucose is oxidized by GDH(oxy and in the process converts GDH(X) to GDH(red), as shown in Eq. 4 below. Note that GDH(,) refers to the oxidized state of GDH, and GDH(rcd) refers to the reduced state of GDH. G:\Patent Specifications - Word\19469 Speci-do16/09/11 -28 Eq. 4 D-Glucose + GDH( 0 ) 4 Gluconic acid + GDH(red) Next, GDH(md) is regenerated back to its active oxidized state by ferricyanide 5 (i.e. oxidized mediator or Fe(CN) 3 -) as shown in Eq. 5 below. In the process of regenerating GDH( 0 ,), ferrocyanide (i.e. reduced mediator or Fe(CN) 6 4 -) is generated from the reaction as shown in Eq. 5: Eq. 5 GDHred) + 2 Fe(CN) -- > GDH( 0 X) + 2 Fe(CN)64 10 In general, the rate of glucose consumption based on Eq. 4 and Eq. 5 is faster for control solution than for blood. Typically, the control solution is less viscous than blood causing the reaction rate of Eq. 4 and Eq. 5 to be faster for the control solution. Further, the reaction rate is faster for the control solution because a portion 15 of the glucose present in the blood sample must diffuse out of the red blood cells to participate in Eq. 4. This extra step of glucose diffusion out of the red blood cells slows down the reaction rate to some measurable degree. FIG. 9 shows that the reaction rate for blood is slower than for control solution as evidenced by the fact that the general absolute slope value (e.g., between about 1.2 and about 4 seconds) 20 for the second current transient is less for the blood sample. Because of the faster reaction rates in the control solution as compared to blood, the residual reaction index for control solution will generally be lower than for blood. The residual reaction index can be a number that is related to the percent of glucose that has not been consumed. In one embodiment, a relatively low residual 25 reaction index can indicate that the reactions of Eq. 4 and Eq. 5 are close to completion. In contrast, a relatively high residual reaction index will indicate that the reaction is not close to completion. For example, the residual reaction index can be an absolute ratio of a current value of a third current transient divided by a current value of the second current transient, as shown in Eq. 6: 30 Eq. 6 abs(4.1) ( (3.8) For the denominator of Eq. 6, the current value at 3.8 seconds for the second G:\Patent Specifications - Word\19469 Speci.doc6/09 1 11 -29 current transient was used. The time of 3.8 seconds was chosen empirically, however, one skilled in the art will appreciate that other current values can be used. In one embodiment, a current value at about the end of the second current transient is chosen. During the second test potential time interval T 2 , the reduced mediator is 5 oxidized at the second electrode 164. The magnitude of the current values measured during second test potential time interval T 2 can be ascribed to the amount of ferrocyanide generated by reagent layer 72 at the first electrode 166 and then diffused to the second electrode 164. It is assumed that the reagent layer 72 remains close to the first electrode 166 after it dissolves in blood. As a result, most of the 10 ferrocyanide that is oxidized by the second electrode 164 necessarily had to diffuse from first electrode 166. For the numerator of Eq. 6, the current value at about 4.15 seconds was used. Other current values from the third current transient can be chosen, however current value at about the beginning of the third current transient are preferred. During the 15 third test potential time interval T 3 , the reduced mediator is oxidized at first electrode 166. The magnitude of the current values measured during the second test potential time interval T2 can be ascribed to the amount of ferrocyanide generated by the reagent layer 72 at the first electrode 166 and did not diffuse sufficiently far away from the first electrode 166. As a result of the reagent layer 72 remaining close to 20 the first electrode 166, the magnitude of the current values for the third current transient will be generally larger than the second current transient. In addition, the third current transient will also be larger in magnitude than second current transient because the reagent layer 72 will have had more time to generate ferrocyanide. Thus, the absolute ratio as shown in Eq. 6 will be larger if the glucose reaction is still 25 far from completion at the time of the measurement. In another embodiment, a residual reaction index can be used, as shown in Eq. 7 below. The residual reaction index can increase to indicate that the reactions of Eq. 4 and Eq. 5 are closer to completion and decrease to indicate that the reactions are further from completion. It should be noted that Eq. 6 has a residual reaction 30 index that ranges from about 1 to infinity and that Eq. 7 has a residual reaction index that ranges from about zero to about 1. Under certain circumstances, Eq. 7 may be a better discriminator for control solution than Eq. 6. For example, the residual G:\Pateni Specifications - Wod\9469 Specidol16/D9/1 I - 30 reaction index can be an absolute ratio of a current value of second current transient divided by a current value of the third current transient, as shown in Eq. 7: Eq.7 abs i(38) i(4.15) 5 FIG. 10 is a chart showing a non-linear relationship between the estimated percent (%) of substrate consumed and the residual reaction index of Eq. 6 for blood samples having various hematocrit levels and for control solution (diamonds = 25% hematocrit blood, squares = 42% blood, triangles = 60% hematocrit blood, x = 10 control solution). FIG. 10 illustrates that the residual reaction index is relatively high when the % of substrate consumed is low and that the residual reaction index is relatively low when the % of substrate consumed is high for a given sample Co type/hematocrit value. The % of substrate consumed is estimated from a ratio where C, is an estimated substrate concentration at the electrode surface and YSI is 15 the substrate concentration using a standard reference technique. The term C 0 is derived using the following Eq. 8: Eq. 8 C= 2FAD 20 In this equation, L is the distance between the first electrode 166 and the second electrode 164, F is Faraday's constant, A is the area of first electrode 166, and D is the diffusion coefficient. Further details regarding Eq. 8 can be found in U.S. Patent No. 6,284,125, the entirety of which is hereby incorporated herein by reference. 25 FIG. 11 is a chart showing a relationship between the interferent index and a residual reaction index for a plurality of blood samples and control solution samples. By plotting the interferent index on the X-axis and the residual reaction index on the Y-axis, a segregation between blood and control solution can be observed. A discrimination line can be drawn to determine if the sample is either a control 30 solution or blood. In this embodiment, the interferent index is i(0.3) +i(0.35) and G:\Patent Specifications - Word\19469 Speci.doc16/09/11 -31 i(4.1 5) the residual reaction index is abs - i(318 It should be noted that the times (e.g., 4.15, 3.8) at which the current values were selected for the residual reaction index, were found empirically. A large number of current ratios were evaluated for their ability to discriminate between 5 blood and control solution samples. The ratio shown in either Eq. 6 or Eq. 7 was selected because it was found to produce significant separation between blood and control solution samples. A discrimination line was derived to allow the test meter to determine whether the sample was a control solution or blood. For all of the control solution 10 samples tested, the interferent index was plotted versus the residual reaction index. Next, a line was calculated using linear regression for control solution samples. After calculating an equation for the line, the perpendicular bias between each data point and the line was calculated. The perpendicular bias represents the shortest distance between the data point and the line as opposed to a vertical bias that is 15 commonly calculated. A standard deviation was determined for all of the perpendicular biases (SDperp). Lastly, the line was shifted 3* SDyer, units towards the data points for the blood group. The reason for this approach is that the data for the control solution group show very little scatter and therefore the "99% confidence limit" of the control solution group is well-defined. 20 In the method described herein, the information obtained from this statistical analysis of the residual reaction index and the interferent index can be used by the test meter to distinguish a control solutions from blood samples. The test meter can calculate the interferent index and residual reaction index and use these values in association with the derived discrimination line (or an equation representing the 25 discrimination line) to distinguish control solutions from blood samples. For the situation in which the test potential waveform of FIG. 12 is employed, an alternative algorithm for discriminating between control solution and blood may be used. The alternative algorithm includes using the interferent index of Eq. 3 and the residual reaction index of Eq. 7. FIG. 14 illustrates a chart showing the 30 relationship between the interferent index and a residual reaction index for a plurality of blood samples and control solution samples using the alternative algorithm. In jAPatent Specifications - Wordi19469 Speci doc6/09/11 -32 FIG. 14, blood samples and control solution samples were tested over a temperature range of about 5 degrees Celsius to about 45 degrees Celsius. Additionally, the blood samples had a glucose concentration range of about 20 mg/dL to about 560 mg/dL and a hematocrit level range of 0% to about 60%. FIG 15 illustrates another 5 chart where more test strips (about 27,400) were tested using the alternative algorithm at room temperature only. By plotting the interferent index on the X-axis and the residual reaction index on the Y-axis, a segregation between blood and control solution can be observed in FIGS. 14 and 15. A discrimination criterion can be used to determine if the sample is either 10 control solution or blood based on the interferent index of Eq. 3 and the residual reaction index of Eq. 7. For example, the interferent index of Eq. 3 may be compared to a pre-determined threshold and the residual reaction index of Eq. 7 may be compared to a pre-determined threshold equation. The pre-determined threshold may be about 10 rnicroamperes. The pre-determined threshold equation may be 15 based on function using the interferent index. More specifically, the pre-determined threshold equation can be Eq. 9. K* i(t 10 Eq. 9 abs i(3.8) 0 i(4.15) it t=0.05 20 The term K can be a constant such as, for example, about 0.2. Thus, the alternative algorithm can identify a sample as blood if j i(t)> 10 and if 14=.05 K*si(t) 8 -10 abs i(3.8) - o0 ' , else the sample is control solution. i(4.1 5)) i=0.05 EXAMPLES 25 Example 1: The following provides the preparation of the control solution which was utilized to generate the data of FIGS. 7 and I1. This preparation is non-limiting as G-\Pateni Specifications - Word\19469 Speci.doc 16109/ 1 - 33 various other preparations and/or control solutions can be utilized with the currently disclosed system and method. Citraconic acid Buffer Component 0.0833 g Dipotassium citraconate Buffer Component 1.931 g 5 Methyl Paraben Preservative 0.050 g Germal II Preservative 0.400 g Dextran T-500 Viscosity Modifier 3.000 g Pluronic 25R2 Wicking Agent 0.050 g 1-[(6-methoxy-4-sulfo-m-tolyl)azo]-2-naphthol- 6 -sulfonic acid disodium salt 10 Dye (FD&C Blue No. 1) 0.100 g D-Glucose Analyte 50, 120, or 525 mg Deionized Water Solvent 100 g First citraconic buffer pH 6.5±0.1 was prepared by dissolving required quantities of citraconic acid and dipotassium citraconate in deionized water. Next, 15 methyl paraben was added and the solution was stirred until the preservative was fully dissolved. Subsequently Dextran T-500, Germal II, Pluronic 25R2 and 1-[(6 methoxy-4-sulfo-m-tolyl)azo]-2-naphthol-6-sulfonic acid disodium salt were added sequentially, following complete dissolution of the previously added chemical. At this point, the pH of the control fluid was verified, followed by addition of the 20 requisite quantity of glucose to obtain a low, normal or high glucose level of the control fluid. After the glucose was dissolved completely, the control fluid was left at room temperature overnight. Finally, the glucose concentration was verified using a Model 2700 Select Biochemistry Analyzer manufactured by Yellow Springs Instrument Co., Inc. The dye used in this control solution has a blue color, which 25 reduces the possibility of a user confusing the control solution with blood. Example 2: Some people (e.g., young people attempting to deceive parents or doctors) will load a sensor with Gatorade* as opposed to blood to give the impression that 30 their glucose is under control. The following experiment was performed to determine if the currently disclosed method and sensor could be utilized to distinguish Gatorade* from blood. G:\Paten Specifications - WorJ\9469 Speci.doc16/09/I1 -34 Five (5) different flavors of Gatorade* were tested. The sensor classified all 5 flavors as a control solution (mean glucose = 264 mg/dL; CV = 6.7%). Thus, the sensor can be used to distinguish between Gatorade* and blood. One skilled in the art will appreciate further features and advantages of the 5 presently disclosed system and method based on the above-described embodiments. Accordingly, the presently disclosed system and method are not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety. 10 Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 15 The reference to any prior art in this specification is not and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge. M:\Graham\CIare\Speci & Amndmts\19469 Speci Amndmls.Docx, 29/08/12
Claims (20)
1. A method for distinguishing between a blood sample and an aqueous non blood sample, the method including: 5 (a) introducing a sample into an electrochemical cell, the electrochemical cell including: (i) two electrodes in a spaced apart relationship; and (ii) a reagent; (b) applying a first test potential, having a first polarity, between the 10 electrodes, and measuring cell current; (c) summing at least three current values measured during the first test potential to generate an interferent index; and (d) using the interferent index to distinguish between a blood sample and an aqueous non-blood sample, 15 wherein said summing step is represented by an equation, the equation being i = ji(t), where isum is the summation of current values and t is a time. t=0.05
2. A method according to claim 1, wherein the aqueous non-blood sample is a control solution. 20
3. A method according to either claim I or claim 2, further including the step of measuring a concentration of an analyte.
4. A method according to claim 3, wherein if the sample is found to be an 25 aqueous non-blood sample the analyte concentration associated with the aqueous non-blood sample is flagged.
5. A method according to any one of claims 1 to 4, wherein step (d) further includes using statistical classification to determine if the sample is an aqueous non 30 blood sample or a blood sample. M:\Grahan\Clare\Speci & Amndmts\19469 Speci Amndmts.Docx, 29/08/12 -36
6. A method according to any one of claims 1 to 4, wherein step (d) further includes comparing the interferent index to a pre-determined threshold value.
7. A method according to claim 6, wherein the pre-determined threshold value 5 is about 10 microamperes.
8. A method according to claim 6, wherein the pre-determined threshold value is calculated by a pre-determined threshold equation, the equation being K*( i(t))-10 1=0.05 1=0.05 10
9. A system for distinguishing between a blood sample and an aqueous non-blood sample, the system including: (a) a test strip including electrical contacts for mating with a test meter and an electrochemical cell including: 15 (i) a first electrode and a second electrode in a spaced apart relationship; and (ii) a reagent; and (b) a test meter including a processor adapted to receive current data from the test strip, and data storage containing discrimination criteria for distinguishing a 20 blood sample from an aqueous non-blood sample; wherein the discrimination criteria is a first reference value, wherein the test meter distinguishes a blood sample from an aqueous non-blood sample by comparing the first reference value to a pre-determined threshold value, and wherein the first reference value is an interferent index calculated based upon a 25 summation of current values measured during a first current transient, the summation being represented by the equation ism i(t), where isum is the summation of 1=0.05 current values and t is a time.
10. A system according to claims 9, wherein the pre-determined threshold value M:\Graham\Clare\Speci & Amndmts\19469 Speci Amndmts.Docx. 29/08/12 - 37 is about 10 microamperes.
11. A system according to either claim 9 or claim 10, wherein the discrimination criteria further includes a second reference value; and 5 wherein the test meter distinguishes a blood sample from an aqueous non blood sample by comparing the second reference value to a pre-determined threshold equation in addition to comparing the first reference value to the pre-determined threshold value. 10
12. A system according to claim 11, wherein the pre-determined threshold equation is a function of the first reference value.
13. A system according to either claim 11 or claim 12, wherein the second reference value is a residual reaction index that includes an absolute ratio of a current 15 value of a second current transient divided by a current value of a third current transient.
14. A system according to any one of claims 11 to 13, wherein the pre-determined threshold equation includes the residual reaction index being less than K * (i(t)) - 10 20 t=. 5 1=0.05
15. A system according to any one of claims I to 9, wherein the discrimination criteria includes an empirically derived discrimination line. 25
16. A system according to any one of claims 9 to 15, wherein the test meter is configured to measure a concentration of an analyte.
17. A system according to claim 16, wherein the system is configured to flag an analyte concentration of an aqueous non-blood sample. 30 M:\Graham\Clare\Speci & Amndmts\19469 Speci Amndmts.Docx. 31/08/12 - 38
18. A system according to any one of claims 9 to 17, wherein the aqueous non blood sample is a control solution.
19. A method according to claim 1, substantially as hereinbefore described with 5 reference to the accompanying Figures.
20. A system according to claim 9, substantially as hereinbefore described with reference to the accompanying Figures. M:\Graham\CIare\Speci & Amndmts\19469 Speci Amndmts.Docx, 29/08/12
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5660791A (en) * | 1996-06-06 | 1997-08-26 | Bayer Corporation | Fluid testing sensor for use in dispensing instrument |
| WO2006110504A1 (en) * | 2005-04-08 | 2006-10-19 | Bayer Healthcare Llc | Oxidizable species as an internal reference in control solutions for biosensors |
| AU2007201377A1 (en) * | 2006-03-31 | 2007-10-18 | Lifescan, Inc. | Systems and methods of discriminating control solution from a physiological sample |
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2011
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Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5660791A (en) * | 1996-06-06 | 1997-08-26 | Bayer Corporation | Fluid testing sensor for use in dispensing instrument |
| WO2006110504A1 (en) * | 2005-04-08 | 2006-10-19 | Bayer Healthcare Llc | Oxidizable species as an internal reference in control solutions for biosensors |
| AU2007201377A1 (en) * | 2006-03-31 | 2007-10-18 | Lifescan, Inc. | Systems and methods of discriminating control solution from a physiological sample |
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