DE9422380U1 - Biosensor measuring device with error protection procedures to avoid incorrect displays - Google Patents

Description

EP94 916 758.9

PCT/US 94/05323 = WO 94/29706

German translation = Gbm-Anmeldung (branch)

RD 4242/OG/DE

Biosensor measuring device with error protection procedures
to avoid incorrect displays

Field of application of the invention

The invention relates to biosensor measuring devices which
Use disposable test strips, and especially
Failure protection systems and procedures (fail/safe systems
and procedures) designed to prevent biosensor measuring devices
show incorrect results.

Background of the invention

Biosensor measuring devices that use disposable test strips are widely used. They are used to determine the concentration of various analytes (such as glucose and cholesterol) in the blood. As long as the user follows the instructions for use for the respective measuring device carefully, the measured results are usually reliable. However, the user often does not take the necessary care when using the test strip or the measuring device, which results in an incorrect reading. Therefore, the manufacturers

· ··· ··■■

of measuring instruments to reduce the potential for errors when using the measuring instruments.

However, if there is a manufacturing defect in a biosensor measuring device or in the test strips, incorrect measurement results will occur even when used correctly. Although great care is taken in the manufacture of such measuring devices and test strips, there is a need to integrate analytical procedures into the measuring device that detect malfunctions of the measuring device, irregularities in the test strips and operating errors in order to avoid incorrect analysis values.

A number of descriptions of biosensor measuring devices are known from the prior art in which disposable test strips are used. US Patent 5,108,564 (Szuminsky et al.) describes a biosensor measuring device which is used to measure glucose concentrations in the blood. The device is based on a reaction in which glucose in the presence of an enzyme catalyzes a reaction of potassium ferricyanide to potassium ferrocyanide. After the reaction has been completed, a voltage is applied to a reaction zone which leads to a reversal of the reaction, generating a small but measurable current flow. This current is called the Cottrell current. Depending on the concentration of glucose in the reaction zone, it follows a predetermined curve during the reverse reaction. The measured value of the Cottrell current is converted into a measure of the glucose concentration. The meter also measures the impedance in the reaction zone and determines when a blood sample has been placed therein by detecting a sudden change in current flow. At this point the incubation period begins, after which a potential is applied to the reaction zone and the Cottrell current is measured.

European Patent Application 0471986 A2 by Tsutsumi et al. describes a blood glucose measuring system using disposable test strips. The presence of a blood sample is detected by measuring the resistance between a pair of electrodes. A plurality of sample-like strips are also used, each of which has a specified resistance value that distinguishes it from other strips. Each of these strips has a specific application, i.e. it is used for adjusting the device, compensating for a measurement error, for calibration, etc.

U.S. Patent 4,999,582 (Parks et al.), which is also the assignee of the present application, describes a biosensor electrode excitation circuit that determines whether a test strip has been properly inserted into the meter and determines whether the contact resistance is acceptable for at least one electrode on the test strip.

U.S. Patent Application Serial No. 07/451,309, filed December 15, 1989, entitled "Biosensing Instrument and Method" (White), assigned to the same assignee as the assignee of the present application, describes a biosensing meter that uses the "Cottrell" curve to determine glucose concentration. In this meter, the current flow is proportional to the concentration of an analyte in the test cell. However, if there is something wrong with the test cell, the resulting current may have no relationship to the concentration of the analyte. According to White, there is a relationship that can be used to determine whether the current flow through a reaction zone is actually the

Cottrell relation. In particular, it has been found that for the measurement curves of all analysis concentrations, the ratio of the square roots of successive measurement times is approximately reciprocal to the ratio of the Cottrell current measurement values at the same measurement times. If the ratios agree (within certain limits) for several successive measurement periods, the measuring system follows the Cottrell relation. If the ratios do not agree, the measurement is not taken into account.

US Patent 4,940,945 (Littlejohn et al.) describes an interface circuit for use in biosensor measuring devices. It employs a disposable element that includes a pair of electrodes between which the resistance is measured. The circuit detects the presence of a liquid sample by measuring the initial resistance and also determines the amount of liquid in the element.

US Patent 4,420,564 (Tsuji et al.) describes a blood glucose analyzer that uses a reaction cell that includes a sensor with a fixed enzyme membrane and a measuring electrode. The Tsuji et al. system uses a number of error protection procedures. One of these is to determine whether the reaction is taking place within specific defined temperature limits. A second procedure determines whether a reaction current is within a predetermined range.

The biosensor instruments mentioned in the above references can determine when a biological sample is placed in the reaction zone. However, they do not address the problem of how to proceed when the

Amount of sample is insufficient to completely wet the enzymatic reactants present in a reaction zone. While a test can be performed to determine whether a reaction follows the Cottrell relationship (as described, for example, in the above-referenced White patent application), it would be desirable if, in addition to this test, confirmatory tests could be performed to ensure that the reaction does indeed follow the Cottrell relationship.

Therefore, it is an object of the invention to provide a biosensor measuring device having means for performing a plurality of error protection tests during the analysis of a biological sample.

Another object of the invention is to provide a biosensor measuring device with means that can determine whether an appropriate amount of sample has been applied to the reaction zone of the test strip.

Finally, it is also an object of the invention to provide means which can determine whether a biological sample reacts according to the Cottrell relationship during the course of the reaction and which, if this is not the case, cause the resulting measured values to be disregarded.

Summary of the invention

A biosensor measuring device contains a test strip that contains an electrically isolated measuring electrode and excitation electrode as well as a reaction zone connecting them.

After a drop of biological sample fluid is placed in the reaction zone, a number of error protection tests are performed. An electrical circuit performs a drop quantity test which determines the amount of drop placed in the reaction zone. The circuit determines whether a drop has been placed in the reaction zone. After a delay period, it also measures the value of a test current to determine if the amount of drop is sufficient to hydrate the reactants in the reaction zone. The circuit then proceeds to measure the delta change in current at successive measurement times during the reaction. This test determines the difference between successive current readings during a measurement period. If each subsequent measurement is not less than one delta value below the previous measurement, the current is determined to be non-monotonically decreasing and the test is aborted. After the measurement time has elapsed, a test is carried out in which the sum of the current measurements is determined, whereby a processor calculates the linear sum of all current measurements of the test and the ratio between this sum and the last current measurement. If the ratio corresponds to a previously determined constant for the Cottrell relationship, it is ensured that the measurements follow the Cottrell relationship.

Description of the characters

Figure 1 shows a plan view of a test strip; Figure 2 shows a circuit diagram/block diagram of a biosensor measuring device according to the invention;

Figure 3 shows a curve diagram of both the excitation electrode of the test strip according to Figure

·

1 applied excitation voltage as well as the resulting measuring current measured at a measuring electrode of the test strip; Figure 4 shows an enlarged view of the waveform of the measuring current which is obtained at the beginning of the detection of a

a drop of an analyte;

Figure 5 shows an enlarged view of a plurality of current measurements taken during the measurement period, which follow a previously known Cottrell relationship.

Figures 6 and 7 show high level flow charts illustrating the fault protection tests performed by the electrical circuit of Figure 2 by determining the amount of drop, delta and sum of

Current measurements.

Detailed description of the figures

The test strip 10 shown in Figure 1 has two electrodes, namely a measuring electrode 12 and an excitation electrode 14. These electrodes are fixed to a polymer layer 16. Above them is a cover layer 18 with openings 20 and 21 through which parts of the electrodes 12 and 14 are accessible. The opening forms a sample receptacle and defines a reaction zone between the measuring electrode 12 and the excitation electrode 14. A layer of enzymatic reactants (not shown) lies above the electrodes 12 and 14 in the opening 20 and forms a substrate for receiving a liquid containing the analyte. In this example, it is assumed that the sample containing the analyte is a drop of blood in which the glucose content is to be determined. The opening 21 exposes the electrodes

12 and 14 are free so that an electrical connection can be made when the test strip 10 is inserted into a biosensor measuring device.

The biosensor meter 22 shown schematically in Figure 2 has a window (not shown) for receiving the test strip 10 and for establishing an electrical connection between the excitation electrode 14 and a pair of terminals A and B and between the measuring electrode 12 and a pair of terminals C and D. The excitation electrode 14, when continuously and correctly inserted, establishes an electrical connection between the terminals A and B. Likewise, when the test strip 10 is correctly inserted, the measuring electrode 12 forms a short circuit between the terminals C and D. The terminals A, B, C and D are arranged spaced apart from one another within the biosensor meter 22. This makes it possible to determine whether a test strip 10 has been correctly inserted into the biosensor meter 22 and whether its electrodes indicate correct impedance values. Once it is indicated that a test strip (with a suitable excitation electrode and measuring electrode) has been properly inserted, a drop of blood can be applied to the sample receptacle of the test strip 10, after which measurements can be taken.

The excitation voltage source V _e of an excitation voltage source 23 is applied to the excitation electrode 14 via the operational amplifier 24 and the connection contact A. A second input signal is fed from the connection contact B to the operational amplifier 24 via the line 26. This input signal is also fed to an analog/digital converter (A/D converter) 28, the digitized output signal of which is in turn fed to a bus 30

is applied. On the measuring side of the biosensor measuring device 22, the connection contact C is connected to one input of the operational amplifier 32. The second input of the operational amplifier 32 is connected to a reference potential. The output signal of the operational amplifier 32 is passed to the bus 30 via an A/D converter 34.

The connection contact D is connected via a current conductor 36 and a multiplex switch 38 to an A/D converter 40, the output signal of which is in turn applied to the bus 30. A supply voltage V is connected via a resistor to an input of the A/D converter 40. When the biosensor measuring device 22 is switched on for the first time, the switch 38 is closed in order to enable a check to be made as to whether the measuring electrode 12 has been inserted correctly. As soon as this has been determined, the switch 38 is opened, whereby the input signal to the A/D converter 40 is interrupted.

A microprocessor 42 and associated display 44 are connected to bus 30 and control the overall operation of the biosensor meter 22. The microprocessor 42 also controls, via a line 46, the excitation voltage which is supplied from the excitation voltage source 23 through the operational amplifier 24 to the terminal A. A plug-in ROM memory chip 48 can be connected to the bus 30. It enables the entry of constants and other test parameters for a group of test strips 10.

In the co-pending patent application No.

by White et al. entitled "Biosensing Meter with Disposable Sample Strips and Check Strips for Meter Quality Determinations"

the quality of the biosensor measuring device), submitted on
same day as the present application (reference number
of the lawyer 058-924262 NA), it is described in detail how the biosensor measuring device 22 ensures the correct insertion of the
Test strip 10 and the continuous connection of the
Excitation electrode 14 and measuring electrode 12 are checked.
The content of this patent application by White et al. is
by reference to the content of the present application
made.

The determination by the microprocessor 42 that a test strip has been correctly inserted and the measuring electrode 12 and the excitation electrode 14 have the required continuous connection causes the excitation voltage source 23 to conduct an excitation potential V _e to the operational amplifier 24 and thus to the connection contact A. The course of the excitation potential V _e is shown in Figure 3 as curve 60. First, a high potential value 62 is applied to the excitation electrode 14 and the leakage current between the excitation electrode 14 and the measuring electrode 12 is measured. If the leakage current is within an acceptable range, the microprocessor 42 indicates (on the display 44) that the user can apply a drop of blood to the sample holder 2 0. When the drop of blood is applied, an immediate decrease in resistance (ie an increase in current) is measured between the electrodes 12 and 14. The resulting output signal of the operational amplifier 32 is shown as pulse 64 of the signal curve 66. Figure 4 shows an enlarged view of the pulse 64.

If the pulse 64 exceeds a first limit value 68,
the microprocessor 42 determines that a drop of blood has been detected. The limit value 68 is set low,
to quickly determine that a blood sample

be applied to the test strip 10 and thereby make a clear determination that an incubation period has begun. When the limit value 68 is exceeded by the pulse 64, the microprocessor 42 starts a delay time d, at the end of which (at time 70) a second measurement of the curve 64 is carried out. The delay time d is adjusted so that the blood drop can wet the entire area of the sample holder 20. If the current measured value determined at time 70 is below a sample quantity limit value 72, the test is aborted because the amount of blood drop is then insufficient to ensure complete hydration of the enzymatic reactants within the sample holder 20. However, if the voltage (current) at time 70 exceeds the sample quantity limit value 72, the test is continued.

Shortly thereafter, the microprocessor 42 causes the excitation potential V _e of the excitation voltage source 23 to be separated from the connection contact A (curve section 74 in Figure 3). The curve section 74 represents the incubation time. It extends over a sufficiently long period of time for an enzymatic reaction to take place between the blood drop and the enzymes in the sample holder 2 0 .

After the incubation time has elapsed, an excitation potential V _e (curve section 76 in Figure 3) is again applied to the connection contact A, which triggers a reverse reaction in the sample holder 20. The resulting current (curve section 78) is measured by means of the measuring electrode 12. Figure 5, in which the curve 78 is shown enlarged, shows a classic Cottrell relationship as it is traversed by the current flow during the above-mentioned reverse reaction. In Figure 5, the measuring current

• φ ·

against elapsed time. As is known in the art, the curve 78 shifts either upward or downward depending on the glucose concentration. During the curve portion 78, the microprocessor 42 records a plurality of current measurements, each taken at a certain time interval k. These measurements enable glucose determination and are also used to ensure that the curve portion 78 follows the Cottrell relationship.

If it is assumed that the glucose concentration is to be determined, the sample receptacle 20 contains the following reactants: an enzyme, an electrolyte, a mediator, a film former and a buffer. The enzyme can be, for example, glucose oxidase (or glucose dehydrogenase), the buffer can be organic or inorganic; the electrolyte can be potassium chloride or sodium chloride; the mediator is preferably potassium ferricyanide and the film formers can be gelatin and propiofin. If the test strip is to be used to determine the concentration of cholesterol, the enzyme is preferably cholesterol oxidase with or without the addition of a cholesterol esterase. The buffer is preferably inorganic and contains an electrolyte such as potassium chloride or sodium chloride. In this case two mediators are used, namely ferricyanide and quinones, which are incorporated into a gelatin film as mentioned above.

Since the chemistry used in such a system is well known in the art, it will not be described in detail here. Suffice it to say that a glucose determination is carried out by first placing a blood sample in the sample well 2 0. The glucose contained in the sample causes a forward reaction of

•♦

Potassium ferricyanide to potassium ferrocyanide. The forward reaction is completed within an incubation period. The subsequent application of a voltage (curve section 76) to the excitation electrode 14 leads to a small current flowing through the measuring electrode 12, which results from the reverse reaction from potassium ferrocyanide to potassium ferricyanide. The electron current during the reverse reaction (curve section 78) is recorded and measured.

Referring to Figures 6 and 7, the operation of the measuring device shown in Figure 2 is described below. First (Figure 6), the microprocessor 42 detects whether a test strip 10 has been inserted by measuring the electrical short circuit of the terminals A and B and the terminals C and D (decision symbol 100). The routine is repeated until the microprocessor 42 detects the presence of a test strip 10. After detecting the presence of a test strip 10 and determining that the contact resistance between the terminals A, B and C, D is within an acceptable range, the microprocessor 42 causes the excitation voltage source 23 to produce an excitation potential value 62 (Figure

3) to the excitation electrode 14. This step occurs before a sample is applied to the test strip 10 and allows the leakage current (if any) between the electrode 12 and the electrode 14 to be measured. At the same time, the microprocessor 42 retrieves a leakage current limit value (I _n ^ _x ) from the ROM memory device 48 and compares this value with a leakage current measurement value i provided by the A/D converter 34 (fields 102 and 104). If the leakage current i is less than the leakage current limit value (i _max ) and thus within ac-

acceptable limits, the procedure continues. Otherwise, the test strip is rejected.

At this point, the microprocessor 42 enters a "drop detection state" in which it is determined when a drop of blood has been placed on the sample well 2 0 and whether the amount of blood drop is sufficient to completely wet the enzymatic reactants therein. First, the microprocessor 42 calls two

γ constants, namely a drop detection limit and a sample amount limit 72, from the ROM memory device 48. The microprocessor 42 then adds the leakage current measurement value i to the drop detection limit to determine the drop detection limit 68 shown in Figure 4 (box 106). Finally, the microprocessor 42 causes the user to be shown on the display 44 that the sample can be added to the test strip.

The microprocessor now goes into a waiting state (with the excitation potential value 62 still applied to the excitation electrode 14). If a drop of blood is placed on the sample holder 2 0, an increase in the current is measured (pulse 64 in Figure 4); if the current measurement value exceeds the limit value 68, it is indicated that a drop of blood has been detected (decision symbol 108). An incubation period now begins, which can last, for example, nine seconds. At the same time, the delay time d set for determining the sample quantity begins, at the end of which a second measurement of the pulse 64 takes place (field 110). If the determined current measurement value exceeds the sample quantity limit value 72, it is ensured that there is sufficient blood in the sample holder 2 0 to hydrate the enzymatic reactants positioned therein (decision symbol 112). If

IN THE

• ■· · »

If there is not enough blood, an error is indicated. If there is enough blood, the procedure continues, with the microprocessor 42 causing the excitation potential V _e to be taken from the terminal A (field 114).

After the incubation time has elapsed, the microprocessor 42 causes the excitation voltage source 23 to apply the excitation potential (curve section 76 in Figure 3) to the connection contact A (field 116). The application of the V _e value 76 reverses the above-mentioned enzymatic reaction and results in a current flow (represented by the curve 78 in Figure 3) being generated between the excitation electrode 14 and the measuring electrode 12. At this point in time, a measuring period begins, during which, as shown in Figure 5, several current measurements 82, 84, 86, etc. (up to measurement 88) are carried out and the current measurement values determined are stored (see Figure 5 and field 118, Figure 7). Each measurement value is determined after a time interval determined by k. Figure 5 shows fourteen such time intervals, with no measurements being taken until the end of the second interval to avoid the measured current values exceeding a maximum current value.

During the time that the current measurements 82, 84, 86, etc. are being determined, a "delta" error protection calculation is performed (field 120) after the second current measurement and then after each subsequent current measurement. It is essentially based on the fact that the curve 78, if it behaves like a Cottrell curve, decreases monotonically and that each subsequent current measurement falls below a previous current measurement by at least a predetermined delta error protection limit. This

The limit value is taken from the ROM memory module 4 8 by the microprocessor 4 2 .

As indicated in field 12 0, the microprocessor 42 determines whether each subsequent current reading i _k is less than or equal to a previous current reading (I ₁ ^ ₁ ) plus the delta fault protection limit. If a previous current reading is determined not to meet this condition because the current trace does not show the expected

&iacgr;&ogr; exhibits monotonous behavior, the user is informed (on the display 44) that the procedure is aborted. The test is repeated for each subsequent current measurement value including the last current measurement value 88. Until then, the procedure continues (decision symbol

124) .

As soon as the current measurement value 88 is determined, the procedure goes to a "current sum" error protection procedure. This error protection procedure, which calculates the sum of the current measurements, again checks the Cottrell behavior during the measurement time. When the last current measurement value 88 is determined, it is multiplied by two constants (ie values) which the microprocessor 42 takes from the ROM memory module 48. The results of this multiplication are used as two limit values which are compared with the sum of all the current measurements 82, 84, 86, etc. determined. If the sum lies between the two limit values, it can be concluded that the curve 78 follows the Cottrell relationship. These steps are illustrated in fields 122, 124, 126 and 128 in Figure 7. The sum of the current measurements I _sum is calculated as follows, as can be seen from field 122:

•'-Sum ~" / , &khgr;-&igr;

k = l

where i _k is one of the m current measurements.

Then, as shown in field 128, it is determined whether I _sum is between the upper and lower limits

in Ku > ¹ Su ₁ TOe > in ^K l

γ where: K ₁ = lower limit constant,

K ₁₁ = upper limit constant, and i _m = last current measurement value.

If the condition specified in field 128 is not met, a signal indicates that the procedure is aborted. However, if the condition is met, the glucose concentration is calculated (field 13 0) and shown to the user on the display.

The basis for the fault protection procedure with determination of the sum of the current measured values results from the following proof.

First consider the ratio r

Σ i

r = £=±—(A)

in the

of all measured current values to the last measured current value.

It must be proven that for any current curve exhibiting Cottrell behavior, the ratio has the same value r _Cottrell , independent of any influencing factors (including glucose concentration).

The characteristic Cottrell behavior can be described by equation (B):

_/4 _, nFAVÖ ^ ₁ , ,

lcottrell (t) = ~!=-r^ ^C ( ^B )

where: η = number of electrons per glucose

molecule released

F = Faraday constant

A= surface of the working electrode

t = time since application of the excitation potential

D = diffusion coefficient

C = glucose concentration

Of the above parameters, η and F are constants. A is determined by the design of the test strip. D and C, which may vary from curve to curve, remain constant for a given test during the measurement of the current curve. Thus, all parameters in equation (B) are constant for a given current curve except for time t.

If the current measurements i _k in equation (A) are replaced by their Cottrell representation in equation (B), the following formula results:

nFAyfÖ

By cancelling the constants in the denominator and numerator, equation (C) can be written as follows:

&Sgr;4

t _m

From equation (D) the following conclusion can be drawn: If a curve exhibits Cottrell behavior, the ratio r in equation (A) calculated from the current measurements of this curve must be equal to the ratio r _Cottrell . Or conversely: If a curve does not exhibit Cottrell behavior, the corresponding ratio r in equation (A) is different from r _Cottrell .

The Cottrell model B, although very accurate, is still a model. Therefore, in practice there may be a slight difference between r and r _Cottrell for a curve with Cottrell behaviour. To take this difference into account, the calculated ratio r is not checked on the basis of an exact agreement with r _Cottrell , but is compared with an upper limit r _CotCrell + ^£ u' ^r cottreii ^and a lower limit r _Cottre ·,.!^! · r _Cottrell , where 8 _U and E ₁ are small numbers.

The following inequality

■^Cottrell ^{+ £} u ^Cottrell > ^ > ^r cottrell ~ ^l ^c

corresponds to the following comparison

* ¹ m ^> 2—1 ^lk> ' ^r Cottrell ~ ^l^cottrell '

k = l

By using

Inequality (E) is rewritten as inequality (F)

m
K ₁₁ > £i _k > K ₁ (F)

k=l

which is used as an error protection test (Figure 7, field 128) .

It should be understood that the foregoing description is only an example of the invention. Numerous alternatives and modifications will be apparent to those skilled in the art without departing from the invention. Accordingly, the invention is directed to all alternatives, modifications and variations that come within the scope of the appended claims.

Claims (10)

1. Biosensor measuring device for receiving a test strip which has electrically isolated excitation and measuring electrodes and a sample zone connecting them in which an analysis reactant is contained, comprising:
an excitation voltage supply device for applying an excitation voltage source to the excitation electrode;
a measuring amplifier connected to the measuring electrode to generate an output signal when a quantity of a biological fluid has been placed in the sample well and to create a current path between the excitation and measuring electrodes;
a processor device connected to the measuring amplifier for first testing whether the output signal exceeds a first limit value and for second testing whether the output signal thereafter exceeds a second, higher limit value, wherein an output signal exceeding the first limit value serves as an indicator of the presence of the amount of sample in the sample receptacle, an output signal exceeding the second, higher limit value serves as an indicator that the amount is sufficient to subsequently carry out an analytical determination on the biological fluid, and the processor device only enables the subsequent determination after the second, higher limit value has been exceeded by the output signal. 2. A biosensor measuring device according to claim 1, wherein the processor performs the second test only after a preset delay time, the delay time being sufficient to allow the amount of biological fluid to substantially wet the analysis reactant. 3. A biosensor measuring device according to claim 1, wherein the amplifier means generates a leakage current signal before the amount of biological fluid has been placed in the sample receptacle, the leakage current signal being an indicator of a leakage current between the excitation electrode and the measuring electrode, further comprising:
a pluggable device including a memory with stored data, the stored data including a leakage current limit, a drop detection limit, and the second limit;
wherein the processor means determines whether the leakage current exceeds the leakage current limit and, if not, adds the leakage current to the drop detection limit to determine the first limit. 4. A biosensor measuring device according to claim 3, wherein the processor means uses the second limit value acquired from the plug-in module to determine whether the amount is sufficient to enable the subsequent determination. 5. A biosensor meter for receiving a test strip including a pair of electrode means and a reaction zone containing an analysis reactant bridging the pair of electrode means, the biosensor meter being arranged to determine whether a current flowing through the reaction zone changes according to a previously known Cottrell relationship, comprising:
a measuring amplifier for determining a plurality of successive measured values of the current flowing in the reaction zone after placing a sample containing an analyte in the reaction zone at a plurality of measuring times, and
processor means for comparing each of the plurality of successive current measurements with immediately preceding current measurements to test whether each of the successive current measurements has a lesser value than the immediately preceding current measurement and, if not, outputting an error signal for the respective test. 6. Biosensor measuring device according to claim 5, further comprising
a pluggable device containing a memory with stored data, one of which is a delta change value, and
wherein the processor device performs the test by summing the delta change value, which is respectively taken over from the plug-in module, with one of the current measurement values and comparing the sum value with another of the current measurement values. 7. A biosensor meter for receiving a test strip including a pair of electrode means and a reaction zone containing an analysis reactant bridging the pair of electrode means, the biosensor meter being arranged to determine whether a current flowing through the reaction zone changes according to a previously known Cottrell relationship, comprising:
a measuring amplifier for determining a plurality of successive current measurements at a plurality of m measuring times after placing a sample containing an analyte in the reaction zone, and
a processor device for summing the successive current measurement values and determining whether a ratio between the sum value and a current measurement value determined at an m-th measurement time lies within a predetermined range of values and, if so, proceeding to a further determination. 8. Biosensor measuring device according to claim 7, further comprising:
a plug-in device containing a memory with stored data, wherein a pair of the data consists of an upper (K _u ) and a lower (K ₁ ) comparison constant, wherein the processor device uses the upper and lower comparison constants to determine the range of values. 9. A system for measuring a current i flowing through a reaction zone of a test cell, the current varying as a function of the concentration of an analyte in the reaction zone in such a way that it follows one of a set of curves whose shape is defined by the Cottrell equation, wherein a procedure for determining whether the current varies according to the Cottrell equation is carried out, comprising the following steps: a) measuring the current i at a plurality of measuring times t _n , t _n+1 , t _n+2 . . . t _m to derive a current value i _n , i _n+1 , i _n+2 . . . i _m ; b) comparing each current value with an immediately subsequent current value to test whether the subsequent current value is smaller by at least a predetermined amount, c) outputting a signal indicating that the current measured in the test cell does not vary according to the Cottrell equation if the test of step b) is not met. 10. A system for measuring a current i flowing through a reaction zone of a test cell, the current varying as a function of the concentration of an analyte in the reaction zone in such a way that it follows one of a set of curves whose shape is defined by the Cottrell equation, wherein a procedure for determining whether the current varies according to the Cottrell equation is carried out, comprising the following steps: a) measuring the current i at a plurality of measuring times t _n , t _n+1 , t _n+2 . . . t _m , to derive a current value i _n , i _n+1 , i _n+2 . . . i _m ; (b) summing the current values in up to and including im and determining whether the ratio of this sum to i _m lies within a predetermined constant range of values and, if not; c) Output of a signal indicating that the current measured in the test cell does not change according to the Cottrell equation.