Meter for determining concentrations of components (eg; glucose) in biological fluid samples (eg; blood)

Background of the Invention

This invention relates to methods and apparatus for improving the accuracy of measurements made with instruments of the type described in, for example, U S. Patents 5,243,516; 5,288,636; 5,352,351, 5,385,846, and 5,508,171 The invention is disclosed in the context of such an instrument, but is believed to be useful in other instruments of this general type as well

There are a number of instruments for the determination of the concentrations of biologically significant components of bodily fluids, such as, for example, the glucose concentration of blood There are, for example, the instruments described inU S Patents 3,770,607,3,838,033,3,902,970,3,925,183,3,937,615, 4,005,002; 4,040,908, 4,086,631; 4,123,701, 4,127,448, 4,214,968; 4,217,196, 4,224,125, 4,225,410, 4,230,537, 4,260,680, 4,263,343, 4,265,250; 4,273,134, 4,301,412, 4,303,887; 4,366,033; 4,407,959; 4,413,628; 4,420,564; 4,431,004, 4,436,094; 4,440,175, 4,477,314, 4,477,575, 4,499,423, 4,517,291, 4,654,197; 4,671,288, 4,679,562, 4,682,602; 4,703,756, 4,711,245, 4,734,184, 4,750,496, 4,759,828, 4,789,804, 4,795,542; 4,805,624; 4,816,224; 4,820,399, 4,897,162, 4,897,173; 4,919,770, 4,927,516, 4,935,106, 4,938,860; 4,940,945; 4,970,145, 4,975,647; 4,999,582; 4,999,632; 5,108,564, 5,128,015, 5,243,516; 5,269,891; 5,288,636; 5,312,762; 5,352,351, 5,385,846, 5,395,504; 5,469,846, 5,508,171, 5,508,203, and 5,509,410 U.S. Patent No. 5,997,817 · European Patent

Specifications. 206,218, 230,472,241,309; 255,291; and, 471,986. and, Japanese Published Patent Applications JP 63-128,252 and 63-111,453. There are also the methods and apparatus described in. Talbott, et al, “A New Microchemical Approach to Amperometric Analysis,” Microchemical Journal, Vol 37, pp 5-12 (1988), Morris, et al, “An Electrochemical Capillary Fill Device for the Analysis of Glucose Incorporating Glucose Oxidase and Ruthenium (III) Hexamine as Mediator, Electroanalysis,” Vol 4, pp 1-9 (1992), Cass, et al, “Ferrocene-Mediated Enzyme Electrode for Amperometric Determination of Glucose,” Anal Chem , Vol 56, pp 667-671 (1984), Zhao, “Contributions of Suspending Medium to Electrical Impedance of Blood,” Biochimica et Biophysica Acta, Vol 1201, pp 179-185 (1994),

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-2Zhao, “Electrical Impedance and Haematocnt of Human Blood with Various Anticoagulants,” Physiol Meas , Vol 14, pp 299-307 (1993), Muller, et al, “Influence of Hematocrit and Platelet Count on Impedance and Reactivity of Whole Blood for Electrical Aggregometry,” Journal of Pharmacological and Toxicological Methods, Vol 34, pp 17-22 (1995), Preidel, et al, “In Vitro Measurements with Electrocatalytic Glucose Sensor in Blood,” Biomed Biochim Acta, Vol 48, pp 897903 (1989), Preidel, et al, “Glucose Measurements by Electrocatalytic Sensor in the Extracorporeal Blood Circulation of a Sheep,” Sensors and Actuators B, Vol 2, pp 257-263 (1990), Saeger, et al, “Influence of Urea on the Glucose Measurement by Electrocatalytic Sensor in the Extracorporeal Blood Circulation of a Sheep,” Biomed Biochim Acta, Vol. 50, pp. 885-891 (1991), Kasapbasioglu, et al, “An Impedance Based Ultra-Thin Platinum Island Film Glucose Sensor,” Sensors and Actuators B,

Vol 13-14, pp 749-751 (1993), Beyer, et al, “Development and Application of a New Enzyme Sensor Type Based on the ElS-Capacitance Structure for Bioprocess Control,” Biosensors & Bioelectronics, Vol 9, pp. 17-21 (1994), Mohri, et al, “Characteristic Response of Electrochemical Nonlinearity to Taste Compounds with a Gold Electrode Modified with 4-Aminobenzenethiol,” Bull Chem Soc Jpn, Vol 66, pp. 1328-1332 (1993), Cardosi, et al, “The Realization of Electron Transfer from Biological Molecules to Electrodes.’’Biosensors Fundamentals and Applications, chapt 15 (Turner, et al, eds., Oxford University Press, 1987), Mell, et al, “Amperometnc Response Enhancement of the Immobilized Glucose Oxidase Enzyme Electrode,” Analytical Chemistry, Vol 48, pp 1597-1601 (Sept 1976), Mell, et al, “A Model for the Amperometnc Enzyme Electrode Obtained Through Digital Simulation and Applied to the Immobilized Glucose Oxidase System,” Analytical Chemistry, Vol 47, pp. 299-307 (Feb 1975), Myland, et al, “Membrane-Covered Oxygen Sensors An Exact Treatment of the Switch-on Transient,” Journal of the Electrochemical Society, Vol 131, pp 1815-1823 (Aug 1984), Bradley, et al, “Kinetic Analysis of Enzyme Electrode Response,” Anal Chem, Vol 56, pp 664-667 (1984),

Koichi,’’Measurements of Current-Potential Curves, 6, Cottrell Equation and its Analogs What Can We Know from Chronoamperometry⁷” Denki Kagaku oyobi Kogyo Butsuri Kagaku, Vol 54, no 6, pp 471-5 (1986), Williams, et al, “Electrochemical-Enzymatic Analysis of Blood Glucose and Lactate,” Analytical

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-3Chemistry, Vol 42, no 1, pp 118-121 (Jan 1970), and, Gebhardt, et al, “Electrocatalytic Glucose Sensor,” Siemens Forsch-u Entwickl-Ber Bd , Vol 12, pp 91-95 (1983) This listing is not intended as a representation that a complete search of all relevant prior art has been conducted, or that no better references than those listed exist Nor should any such representation be inferred

Disclosure of the Invention

According to one aspect of the invention, an apparatus for determining the concentration of a medically significant component of a biological fluid compnses a cell for receiving a sample of the fluid The cell supports a chemistry which reacts with the medically significant component and first and second terminals across which the reaction of the chemistry with the medically significant component can be assessed The apparatus further comprises an instrument having first and second terminals complementary to the first and second terminals, respectively, of the cell Placement of the first and second terminals of the cell m contact with the first and second terminals, respectively, of the instrument permits the instrument to assess the reaction The instrument includes an assessment controller for applying across the first and second terminals of the instrument a first signal, determining a first response of the cell to the first signal, and determining based upon the first response whether to proceed with the determination of the concentration of the medically significant component of the biological fluid

According to another aspect of the invention, an apparatus for determining the concentration of a medically significant component of a biological fluid comprises a cell for receiving a sample of the fluid The cell supports a chemistry which reacts with the medically significant component and first and second terminals across which the reaction of the chemistry with the medically significant component can be assessed The apparatus further comprises an instrument having first and second terminals complementary to the first and second terminals, respectively, of the cell Placement of the first and second terminals of the cell m contact with the first and second terminals, respectively, of the instrument permits the instrument to assess the reaction The instrument includes an assessment controller for applying across the first and second terminals of the instrument a first signal, determining a first correction

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-4value in response of the cell to the first signal, assessing the reaction of the medically significant component with the chemistry and combining the correction value with the result of the reaction assessment to produce an indication of the concentration of the medically significant component in the sample

According to another aspect of the invention, an apparatus for determining the concentration of a medically significant component of a biological fluid compnses a cell for receiving a sample of the fluid The cell supports a chemistry which reacts with the medically significant component and first and second terminals across which the reaction of the chemistry with the medically significant component can be assessed The apparatus further comprises an instrument havmg first and second terminals complementary to the first and second terminals, respectively, of the cell Placement of the first and second terminals of the cell in contact with the first and second terminals, respectively, of the instrument permits the instrument to assess the reaction The instrument includes an assessment controller for applying across the first and second terminals of the instrument a first signal, determining the identity of the sample in response of the cell to the first signal, and producing an indication of the identity of the sample

According to yet another aspect of the invention, a method for determining the concentration of a medically significant component of a biological fluid comprises providing a cell for receiving a sample of the fluid, and providing on the cell a chemistry which reacts with the medically significant component and first and second terminals across which the reaction of the chemistry with the medically significant component can be assessed The method further comprises providing an instrument having first and second terminals complementary to the first and second terminals, respectively, of the cell Placement of the first and second terminals of the cell in contact with the first and second terminals, respectively, of the instrument permits the instrument to assess the reaction The method further compnses providing in the instrument an assessment controller, causing the assessment controller to apply across the first and second terminals of the instrument a first signal, causing the assessment controller to determine a first response of the cell to the first signal, and causing the assessment controller to determine, based upon the first response, whether to proceed

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-5with the determination of the concentration of the medically significant component of the biological fluid

According to a further aspect of the invention, a method for determining the concentration of a medically significant component of a biological fluid comprises providing a cell for receiving a sample of the fluid, and providing on the cell a chemistry which reacts with the medically significant component and first and second terminals across which the reaction of the chemistry with the medically significant component can be assessed The method further compnses providing an instrument having first and second terminals complementary to the first and second terminals, respectively, of the cell Placement of the first and second terminals of the cell in contact with the first and second terminals, respectively, of the instrument permits the instrument to assess the reaction The method further comprises providing in the instrument an assessment controller, causing the assessment controller to apply across the first and second terminals of the instrument a first signal, to determine a first correction value in response to the first signal, to assess the reaction of the medically significant component with the chemistry, and to combine the correction value with the result of the reaction assessment to produce an indication of the concentration of the medically significant component in the sample

According to a further aspect of the invention, a method for determining the concentration of a medically significant component of a biological fluid compnses providing a cell for receiving a sample of the fluid, and providing on the cell a chemistry which reacts with the medically significant component and first and second terminals across which the reaction of the chemistry with the medically significant component can be assessed The method further comprises providing an instrument having first and second terminals complementary to the first and second terminals, respectively, of the cell Placement of the first and second terminals of the cell in contact with the first and second terminals, respectively, of the instrument permits the instrument to assess the reaction The method further comprises providing in the instrument an assessment controller for applying across the first and second terminals of the instrument a first signal, determining the identity of the sample in response of the cell to the first signal, and producing an indication of the identity of the sample

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-6Illustratively, the first signal comprises a signal having an AC component Further illustratively, the first signal comprises an AC signal.

Additionally illustratively, the method of, and apparatus for, determining the correction value, the method of, and apparatus for, determining the identity of the sample, and the method of, and apparatus for, determining whether to proceed with the determination of the concentration of the medically significant component of the biological fluid comprise the step of, and apparatus for, determining the impedance across terminals of the cell

As used herein the term medically significant component means a component that the measurement of the level thereof in a biological fluid from an individual can assist in the assessment of the state of health of that individual. For example, determining the level of such a component may indicate a predisposition to or the presence (or absence) of a disease or condition, or whether that disease or condition is under control.

Brief Description of the Drawings

The invention may best be understood by referring to the following detailed description and accompanying drawings which illustrate the invention. In the drawings:

Fig. 1 illustrates a schematic diagram of a circuit useful in understanding the invention;

Fig 2 illustrates a partly block and partly schematic diagram of an instrument constructed according to the present invention,

Fig 3 illustrates a partly block and partly schematic diagram of another instrument constructed according to the present invention;

Fig. 4 illustrates a partly block and partly schematic diagram of another instrument constructed according to the present invention;

Fig. 5 illustrates glucose concentration results achieved in several forty second glucose concentration determinations with standard glucose test solutions;

Fig 6 illustrates glucose concentration results achieved in several ten second glucose concentration determinations with standard glucose test solutions; and,

Fig. 7 illustrates glucose concentration results achieved in several ten second glucose concentration determinations with standard glucose test solutions

Detailed Descriptions of Illustrative Embodiments

Instruments are known which employ devices such as disposable mediated amperometric cells (sometimes referred to hereinafter as biosensors) which provide, for example, characteristic electrical impedances when treated with biological

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-7fluids, blood or urine for example, having certain corresponding concentrations of biologically significant components, such as, for example, glucose Such measurement systems are known to be susceptible to variations in the temperature of the biological fluids and to interference by the presence m the biological fluids of other components, known and sometimes referred to hereinafter as mterferrents In many cases, these sources of error have effects on the biosensor output of the same order of magnitude as the concentration of the component, measurement of which is sought It may not be possible to develop a biosensor which will measure only the concentration of the component whose concentration is sought in the presence of these sources of error An example of this phenomenon is the hematocrit interference in a biosensor of the type descnbed in U S Patents 5,243,516, 5,288,636, 5,352,351, 5,385,846 and,

5,508,171, with the determination of the glucose concentration of whole blood Since all whole blood contains red blood cells, and since the hematocrit can vary over a fairly wide range in individuals who might wish to rely upon such biosensor testing, the utility of a hematocnt-compensated glucose biosensor is clear

Equally problematic is the sensitivity of many commercially available biosensors to the volume of the dopant biological fluid In the case of glucose concentration of whole blood, for example, many presently available biosensors are sensitive to the volume of blood with which they are doped for determination of glucose concentration Since many of the tests which are presently being conducted using biosensors are being conducted by people who are monitoring, for example, the glucose concentrations of their own blood, the volumes of the blood samples with which the biosensors are doped are not predictable with a great degree of certainty While the careful design of the biosensor itself can prevent some errors, such as undoped biosensors, substantially underdoped biosensors and substantially overdoped biosensors, for example, it cannot practically take into account the full range of doping volume variation

We have discovered that measurement of the real component or the imaginary component, or both, of the AC impedance of an appropriately designed biosensor provides reasonable insight into sample temperature and the concentrations of certain physical and chemical interferrents In biosensors of the general types described in U S Patents 5,243,516, 5,288,636; 5,352,351, 5,385,846, 5,508,171,

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5,437,999, and, U S 5,997,817 , filed December 5, 1997 and assigned ro the same assignee as this application, such physical interferrents include, for example, hematocrit, and such chemical interferrents include, for example, bilirubin, uric acid and oxygen We have discovered that measurement of the real component or the imaginary component, or both, of the AC impedance of an appropriately designed biosensor also provides reasonable insight into the volume of a sample with which the biosensor is doped, and the identity of that sample, that is, whether the sample is a sample of blood or some other bodily fluid, or a sample of some control used, for example, in calibration or troubleshooting of the instrument We have discovered that sample temperature, the concentrations of such physical and chemical interferrents, the identity of the sample and the sample volume can be ascertained at judiciously selected AC frequencies, providing reasonable isolation of the determinations of the effects of sample temperature, interferrent concentrations and sample volume and identity from each other, and thereby increasing the accuracy of, for example, the interferrent effect determinations, and their subsequent correction out of the indicated glucose concentration We have also found that the speeds at which acceptably accurate readings of corrected glucose concentration are obtained can be markedly reduced The appropriately designed biosensor must be able to tolerate the determination of these AC impedances, using, for example, AC signals having peak amplitudes in the range of a few tens of millivolts, without jeopardizing the measurement of the glucose concentration, which the biosensor will perform either before, concurrently with, or after it performs the AC impedance determination

By way of example only, we have determined that in biosensors of the type described inU S. Patents^- 5,243,516; 5,288,636; 5,352,351; 5,385,846;

5,508,171, 5,437,999, and, 5,997,817 , it is possible to employ a lowmagnitude, for example, less than about 40mV rms or so, AC signal in the range of less than about 1Hz to lOKHz or so with no DC offset to compensate for sample temperature, hematocrit, bilirubin concentration, uric acid concentration and oxygen concentration, and to determine identity of the sample with which the biosensor is dosed, and adequacy of dosed blood sample volume for a test for glucose concentration We have determined, for example, that at about 1300Hz, both hematocrit and glucose concentration have relatively little effect on AC impedance,

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-9while sample volume and sample identity have relatively substantially greater, fairly readily ascertainable, effects on AC impedance This provides an ideal way to determine the adequacy of the sample volume with which the biosensor is dosed and the identity of the sample If the sample is determined to be blood, and the sample volume is determined to be inadequate to test meaningfully for hematocnt, glucose concentration, and so on, the test is discontinued and the user is notified of the discontinuance of the test

We have determined that the combined effect of sample temperature and hematocrit can fairly effectively be isolated from other physical and chemical interferrents of interest using frequencies in the range of from about 2KHz to about 1 OKHz So, for example, once the adequacy of the sample volume for test has been established, a 2KHz signal can be applied to the biosensor and the real and imaginary components of impedance of the biosensor/sample system can be determined This indicated impedance can be adjusted by an experimentally determined scaling factor governed by, among other things, the charactenstics of the biosensor and the instrument, and combmed with an indicated glucose concentration to arrive at a glucose concentration compensated for the combined effects of sample temperature and hematocrit

These determinations illustratively are made before the amperometnc determination of the glucose concentration of the blood sample DC offset may be avoided, if necessary, to reduce the likelihood of affecting the amperometric determination of the glucose concentration which, it must be remembered, is going to be conducted subsequently in the illustrated embodiments Similar procedures can be conducted, again in the illustrated embodiments before the amperometric determination of the glucose concentration, to determine the concentrations of other interferrents with chemistry for the glucose concentration determination, such as bilirubin, unc acid and oxygen These determinations are conducted at frequencies at which their effects upon each other and upon other physical and chemical interferrents will be optimally decoupled from each other For example, if, in the chemistry system of the amperometric cell, bilirubin and uric acid are chemical interferrents with each other, a frequency or range of frequencies should be selected for the bilirubin concentration determination, which frequency or range of frequencies is optimally

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unaffected by the concentrations of uric acid and any other physical and chemical interferrents in the sample. Similarly, a frequency must be selected for the uric acid concentration determination which is optimally unaffected by the concentrations of bilirubin and any other physical and chemical interferrents in the sample. In each case, however, the determined impedance is converted either directly or via a concentration determination which can also be displayed to the user or stored in the instrument for future reference, to a correction factor for application to the indicated glucose concentration in order to arrive at a more accurate glucose concentration determination.

The methods and apparatus are believed best understood by consideration of the equivalent circuit of an amperometnc sensor of the type described inU. S Patents: 5,243,516; 5,288,636, 5,352,351; 5,385,846; 5,508,171, 5,437,999, and, U.S. 5,997,817. That equivalent circuit is illustrated in Fig 1. In Fig. 1, a resistor 20 represents the uncompensated resistance of the amperometric cell, a capacitor 22 represents the capacitance attributable to the double layer of charge on the dosed cell with potential applied, a resistor 24 represents the charge transfer resistance of the cell’s chemistry, and a resistor 26 and a capacitor 28 represent the socalled Warburg impedance While the lumped electrical parameter models of other types of amperometric sensors may differ from the model illustrated in Fig 1, similar analyses of those models will yield conclusions similar to those reached here, namely, that the real and imaginary components of the cells’ or biosensors’ electrical impedances provide techniques for determining quantitatively with some reasonable degree of accuracy the effects of interferrent concentrations, sample volume and sample identity on the concentration of a biologically significant component of a sample of a body fluid These conclusions give the instrument and cell designer useful techniques for determining the adequacy of the volume of a sample applied to a biosensor, for determining the identity of the sample, and for correcting the indicated concentration of a biologically significant component of the sample for the concentration(s) of such interferrent(s) so that the effects of the concentration^) of such interferrent(s) can be reduced in the indicated concentration of the biologically significant component of interest to provide more accurate information on the concentration of the biologically significant component of interest.

t i.-.d-uiCi τ// GFhU S i | 1 Αΐιη l

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Blood sample studies analyzing the magnitudes of the real and imaginary components of the impedance of the equivalent circuit of Fig 1 have established that in the range of about lKHz-10KHz, there is very little dependence of the imaginary component of impedance on glucose concentration of the sample, while there is sufficient dependence of the magnitude of impedance on the combination of sample temperature and hematocrit to permit a sample first to be subjected to a lowmagnitude AC signal in this frequency range, the magnitude of impedance to be determined, and a combined sample temperature/hematocrit correction factor to be combined with the indicated glucose concentration determined using the amperometry techniques described in, for example, U S. Patents: 5,243,516, 5,288,636; 5,352,351; 5,385,846, 5,508,171, 5,437,999, and, U S. 5,997,817, to yield a glucose concentration corrected for the combined effects of sample temperature and hematocrit. Similar techniques can be employed to determine sample volume and sample type. The sample volume determination, however, ordinarily will result in a go-no go determination for the remainder of the assay. The sample type detennination ordinarily will determine whether the instrument proceeds to a glucose concentration subroutine including, for example, determination of interfenent correction factors, or to a diagnostic subroutine used to set up the instrument for a later glucose concentration determination

Referring to Fig. 2, a strip connector 30 of the general type illustrated inU. S. Patents· 5,243,516; 5,288,636; 5,352,351, 5,385,846; and, 5,508,171, makes contact between a disposable amperometric sensor cell or biosensor 31 of the general type illustrated in those patents and the instrument 32. The indicated glucose concentration functionality of the instrument 32 is largely as described in those patents

However, additional functions, namely, the correction of the indicated glucose concentration for blood sample volume and the combined effect of sample temperature and hematocrit of the blood sample under test, are implemented in the instrument 32 according to the present invention. It has been established that eight bit analog-todigital (A/D) and digital-to-analog (D/A) computational power permits the instrument

32 to achieve accuracies in the range of about one-half percent or less A first terminal

34-1 of a connector 34 is coupled through a 10ΚΩ resistor to a terminal 36-1 of a switch 36. A terminal 36-2 of switch 36 is coupled to the inverting, or -, input b in,ίuLutU ι Wii. ίtiTY OfS’iCc is f OF H.Z, |

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-12terminal of a difference amplifier 38 An output terminal of amplifier 38 is coupled to a terminal 36-3 of switch 36 A terminal 36-4 of switch 36 is coupled to a terminal 342 of connector 34 DC excitation across the biosensor 31 is established by the output of amplifier 38 For accurate setting of DC excitation of the biosensor 31, feedback from terminal 34-1 is returned to the - input terminal of amplifier 38 Terminals 34-1 and 34-2 contact a common electrode on biosensor 31 for enhanced accuracy of excitation

A terminal 34-3 of connector 34 is coupled to a - input terminal of a difference amplifier 42 An output terminal of amplifier 42 is coupled through a 7 5ΚΩ resistor 44 to the - input terminal thereof The non-inverting, or +, input terminal of amplifier 42 is coupled to the common of the circuit power supply An output terminal of amplifier 42 is coupled to an input terminal of a thirteen bit A/D converter 46 An output port of A/D converter 46 is coupled to an input port of a processor 48 with supporting functions which performs the indicated glucose measurement functions as described in U S Patents 5,243,516, 5,288,636, 5,352,351, 5,385,846, and,

5,508,171 An output port of processor 48 is coupled to an input port of an eight bit D/A converter 50 An output terminal of D/A converter 50 is coupled to the + input terminal of amplifier 38 The functions of components 38, 42, 46, 48 and 50 illustratively, although not by any means necessarily, are embodied in an applicationspecific integrated circuit(ASIC) 52 The remaining, hematocrit compensating and sample volume determining functions of instrument 32 illustratively are embodied in a NEC gPD78054 microprocessor(pP) 54 which also has input A/D and output D/A converting capabilities 56 and 58, respectively In Fig 2, the input A/D and output D/A capabilities 56, 58 are illustrated separately from the processing functions of μΡ 54 for purposes of clarity Terminal 36-4 of switch 36 is coupled to an input terminal of A/D converter 56. The output terminal of amplifier 42 is coupled to an input terminal of A/D converter 56 The output terminal of D/A converter 58 is coupled through a 1 pF capacitor and a 400ΚΩ resistor in senes to terminal 36-1 of switch 36 for AC excitation in this example Here, an AC excitation signal is summed with the DC excitation provided by amplifier 38

The calculations of the real and imaginary components of the AC impedance of the biosensor cell 31 coupled to terminals 34-1, -2 and -3 are made by

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-13excitmg terminal 34-2 of connector 34 at the desired frequency, for example, 1300Hz or 10 KHz, at which the parameter to be determined, be it sample identity or volume or hematocrit, or whatever other parameter is of interest and can be determined this way, varies with sufficient magnitude and phase and is optimally uncoupled from, that is, is not interfered with by, the concentrations of other components of the blood on the cell 31

The calculation of the real and imaginary components of the cell 31 impedance from the AC excitation and response are achieved as follows The eight bit excitation samples are N values E(0), E(1), E(2), E(N-l) These values are developed by sampling the excitation by A/D converter 56 The eight bit response samples are N values V(0), V(l), V(2), V(N-1) These values are A/D converted by A/D converter 56 and returned to the processor function of μΡ 54 Terminal 34-2 of connector 34 provides the common terminal against which these values are referenced A scale factor K accounts for various gam factors involved in excitation and measurement The excitation frequency is F Hz The sample rate is MF, where M illustratively has a value of 5 or more The period between samples is thus 1/MF sec Arrays S(n) and C(n) of sine and cosine values are calculated and stored in program memory in μΡ 54 according to the following relations

S(n) = sin(2mF(n/MF)), n=0 to (N-l)

C(n)=cos(2zF(n/MF)), n=0 to (N-l)

The real and imaginary components of excitation are calculated as follows

N-i

Ere=Σ S(fi)E(n) n=0

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-14The real and imaginary components of response are calculated as

JV-1

Ειηι=Σ C(.ri)E(n) n=0 follows

W-l

Vre=£ S(n)V(ri) n=0

Vtm=X C(n)V(n) «=0 follows

The magnitudes of the excitation and response are calculated as

E=(Ere²+Eim²)^1/2,

V=(Vre²+Vim²)^1/2

The magnitude of the strip impedance can then be calculated:

|Z|=KE/V

The phase of the strip impedance can also be calculated arctan /Vim)- arctan/Enri = Z_ Z \Vre/ \Ere/

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-15504 / t

Thus, a measurement of actual glucose concentration using an instrument 32 of the type illustrated in Fig 2 proceeds as follows A sample of blood is applied to the biosensor 31 Immediately after the instrument 32's electronics detect the deposit of the droplet on the biosensor 31, an AC signal having a frequency of, for example, 1300Hz is applied across terminals 34-2—34-3 of connector 34 and the resulting current is indirectly sampled by μΡ 54 by measuring the excitation and response voltages and using the scale factor to obtain current The impedance magnitude and phase angle are calculated Using these values, a look-up table in the μΡ 54’s program memory is consulted to ascertain the nature of the sample and, if blood, whether there is sufficient volume in the blood sample to proceed with the glucose determination phase of the assay If not, the assay is terminated and this outcome is displayed on the instrument 32's display If there is sufficient volume to continue with the glucose determination, an AC signal at another frequency, for example, 10 KHz, is applied across terminals 34-2—34-3 of connector 34 and the resulting current is sampled by μΡ 54 The impedance and phase angle are again calculated at this second frequency. A second look-up table m the μΡ 54's program memory is consulted for an indicated glucose-to-actual glucose correction factor This correction factor may be a constant, for example, zero, for indicated glucose concentrations less than a first indicated glucose concentration, and variable for indicated glucose concentrations greater than that first indicated glucose concentration, for example In any event, that correction is stored, and the determination of the indicated glucose concentration proceeds generally as described in U S Patents 5,243,516, 5,288,636, 5,352,351, 5,385,846; and 5,508,171, for example. Once the indicated glucose concentration has been obtained, the correction is then retneved and applied to the indicated glucose concentration to arrive at the actual glucose concentration which is displayed on the instrument 32's display and/or stored m the instrument 32's memory

Another embodiment of the invention is illustrated in partly block and partly schematic form in Fig 3 There, an instrument 132 includes a stnp connector 130 of the same general type as strip connector 30 illustrated in Fig 2 Stnp connector 130 is designed to make contact to a biosensor 131. A first terminal 134-1 of a connector 134 is coupled through a 10ΚΩ resistor to a terminal 136-1 of a switch

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136 A terminal 136-2 of switch 136 is coupled to the - input terminal of a difference amplifier 138 An output terminal of amplifier 138 is coupled to a terminal 136-3 of switch 136 A terminal 136-4 of switch 136 is coupled to a terminal 134-2 of connector 134. DC excitation across the biosensor 31 is established by the output of amplifier 138 For accurate setting of DC excitation of the biosensor 31, feedback from terminal 134-1 is returned to the - input terminal of amplifier 138 Terminals 134-1 and 134-2 contact a common electrode on biosensor 31 for enhanced accuracy of excitation A terminal 134-3 of connector 134 is coupled to a - input terminal of a difference amplifier 142 An output terminal of amplifier 142 is coupled through a

7 5ΚΩ resistor 144 to the - input terminal thereof The + input terminal of amplifier

142 is coupled to the common of the circuit power supply An output terminal of amplifier 142 is coupled to an input terminal of a thirteen bit A/D converter 146 An output port of A/D converter 146 is coupled to an input port of a processor 148 with supporting functions which performs the indicated glucose measurement functions as described in U S Patents 5,243,516; 5,288,636; 5,352,351, 5,385,846, and,

5,508,171 An output port of processor 148 is coupled to an input port of an eight bit D/A converter 150 An output terminal of D/A converter 150 is coupled to the + input terminal of amplifier 138 The functions of components 138, 142, 146, 148 and 150 illustratively, although not by any means necessarily, are embodied in an ASIC 152

The real and imaginary components of the AC impedance of the biosensor cell 131 coupled to terminals 134-1,-2 and -3 are calculated by excitation applied between terminals 134-2 and 134-3 of connector 134 at the desired frequencies, for example, by sweeping the low-magnitude AC voltage source 150 through a suitable frequency range of, for example, lHz-100Hzorl0Hz-10 KHz, throughout some portion or all of which the parameter to be determined, be it sample identity, sample volume, sample temperature/hematocnt, oxygen concentration in the sample, or whatever other parameter is of interest and can be determined this way, varies with sufficient magnitude and phase and is optimally uncoupled from, that is, independent from, the concentrations of other components of the sample on the cell 131.

In the embodiment illustrated in Fig 3, this low magnitude AC voltage excitation is summed at a summing junction 152 with an optional DC offset 156 which

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-17may be utilized if it aids the determination of the concentration of the interferent of interest In the illustrated embodiment, the AC voltage and DC offset are both generated under the control of a microprocessor (μΡ) 158 which may be the same μΡ which manages the above-mentioned meter 132 functions, or may be a separate μΡ The μΡ 158 will typically be programmed to sweep the AC voltage source 150 and adjust the DC offset, depending upon which mterferent’s concentration the μΡ 158 is determining In this manner, each mterferent’s concentration may readily be ascertained in the optimum frequency range and at the optimum DC offset for isolation of that particular mterferent’s concentration If μΡ 158 is used to control sweep and offset, a separate external connection 160 need not be provided from the summing junction 152 to the μΡ 158 Smce μΡ 158 is going to determine the frequency response of the cell 31, the frequencies associated with the determined frequency response can be stored in the μΡ 15 8 ’s memory as the frequency response is being determined If some other mechanism is employed m the determination of the frequency response, however, it may be necessary to provide feedback 160 to the μΡ 158 of the output frequency of source 150, as well as the level of the DC offset 156 In any event, isolation of the summing junction 152 and any feedback path 160 from the cell 31 is provided by an operational amplifier 164 whose input is coupled to summing junction 152, and whose output is coupled through a suitably valued resistor into the feedback path of amplifier 138 to drive the cell 31. Similarly, isolation of the cell 31 from the frequency response-determining input of μΡ 158 is provided by an operational amplifier 166 coupled to the output of amplifier 142 Determination of the frequency response of the cell 31 proceeds in known fashion, for example, by fast Founer transform (FFT) or other known μΡ 158-implemented frequency response determining mechanism The frequency response characteristic of the cell 31 is then compared to the stored frequency response characteristic for the specific interferent whose concentration is being determined, an interferent concentration is determined, and an associated correction value for the indicated glucose concentration is determined and either stored for later use in correcting the mdicated glucose concentration or immediately combined with an indicated glucose concentration to achieve a corrected glucose concentration

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-18Again, ordinarily, the instrument 132 will first determine the various frequency responses of the cell 31 m the various optimally uncoupling frequency ranges, with the vanous optimally uncoupling AC amplitudes and with the various optimally uncoupling DC offsets, followed by the determination of the indicated glucose concentration, followed by correction of the indicated glucose concentration for the thus-determined concentrations of the vanous interferrents However, and as previously noted, it may be desirable under certain circumstances and with certain interferrents to have the instrument 132 first determine the indicated concentration of glucose before the concentrations of these interferrents are determined

Another embodiment of the invention is illustrated m partly block and partly schematic form in Fig 4 There, an instrument 232 includes a strip connector 230 of the same general type as strip connector 30 illustrated in Fig 2 Strip connector 230 is designed to make contact to a biosensor 31 A first terminal 234-1 of a connector 234 is coupled to the - input terminal of a difference amplifier 238 An output terminal of amplifier 238 is coupled to a terminal 234-2 of connector 234 DC excitation across the biosensor 31 is estabhshed by the output of amplifier 238 For accurate setting of DC excitation of the biosensor 31, feedback from terminal 234-1 is returned to the - input terminal of amplifier 238 Terminals 234-1 and 234-2 contact a common electrode on biosensor 31 for enhanced accuracy of excitation. A terminal 234-3 of connector 234 is coupled to a - input terminal of a difference amplifier 242 An output terminal of amplifier 242 is coupled through a 8 25ΚΩ resistor 244 to the input terminal thereof The + input terminal of amplifier 242 is coupled to a 1 667V reference An output terminal of amplifier 242 is coupled to an input terminal of a fourteen bit A/D converter 246 An output port of A/D converter 246 is coupled to an input port of a processor 248 with supporting functions which performs the indicated glucose measurement functions as descnbed in U S Patents^- 5,243,516, 5,288,636, 5,352,351, 5,385,846, and, 5,508,171 An output port of processor 248 is coupled to an input port of a thirteen bit D/A converter 250 Amplifier 238 and D/A converter 250 illustratively are integrated into a single device Amplifier 238 has an open circuit shutdown mode, permitting switches 36, 136 of the embodiments illustrated in Figs 23 to be eliminated and thereby simplifying the circuit somewhat Otherwise, the circuit illustrated in Fig 4 functions in much the same way as the circuits illustrated in Figs 2Pnnted from Mimosa

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-193 An output terminal of D/A converter 250 is coupled to the + input terminal of amplifier 238 The functions of components 238, 242, 246, 248 and 250 illustratively, although not by any means necessarily, are embodied in an ASIC 252 The accuracy and resolution of D/A converter 250 and A/D converter 246 enable both AC and DC stnp current measurements and thus a circuit simplification

Again, it should be understood that the physical and chemical design characteristics of a particular cell will, to a large extent, determine the electncal characteristics of that cell Therefore, those physical and chemical design characteristics will, to at least the same extent, determine that cell’s response to each interferent, to different sample types, and to different sample volumes It is not possible to predict, for example, in what frequency range hematocrit’s concentration will be optimally uncoupled from unc acid’s or bilirubin’s without reference to the specific physical and chemical characteristics of that cell Some investigation will be required to determine these optimum frequency ranges However, the investigation will be relatively routine once the physical and chemical characteristics of the cell are known

The reduction in the time required to achieve a compensated indication of the glucose concentration of blood can best be appreciated by referring to Figs 5-7 Fig 5 illustrates glucose concentration results achieved in several forty second glucose concentration determinations with standard glucose test solutions The tests whose results are illustrated in Fig 5 were performed without impedance determination and compensation for the combined effects of temperature and hematocrit descnbed above, but were compensated for temperature and hematocrit using prior art techniques Fig 6 illustrates glucose concentration results achieved in several ten second glucose concentration determinations with standard glucose test solutions The tests whose results are illustrated in Fig 6 were performed without impedance determination and compensation for the combined effects of temperature and hematocrit described above, but again were compensated for temperature and hematocrit using prior art techniques Fig 7 illustrates glucose concentration results achieved in several ten second glucose concentration determinations with standard glucose test solutions The tests whose results are illustrated in Fig 7 were performed usmg impedance determination and compensation for the combined effects of temperature and hematocrit described above

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-20It will be appreciated from a companson of these Figs, that the use of the impedance determination and compensation technique described above permits a reduction by a factor of four in the time required to achieve comparable glucose concentration determination in these test solutions

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