MXPA97002503A

MXPA97002503A - Reagent test stress to determine glucose in the san

Info

Publication number: MXPA97002503A
Application number: MXPA/A/1997/002503A
Authority: MX
Inventors: L Smith John
Original assignee: Lifescan Inc
Priority date: 1996-04-04
Filing date: 1997-04-04
Publication date: 1998-04-01

Abstract

The present invention relates to a reagent test strip for use in an apparatus for determining a glucose concentration in a whole blood sample. The apparatus consists of optical means for detecting light intensity at wavelengths of about 635 nm and about 700 nm reflected from at least a portion of a matrix disposed near one end of the strip, and such array consists of: a) a sample receiving surface for receiving the whole blood sample and passing a portion of it to an opposite test surface thereof, the test surface has a reflectance of approximately 700 nm which, when the test surface becomes wet, undergoes a change which is substantially equivalent to that produced by the absorbance of hemoglobin in the blood, b) a structure that selectively delays the passage of red blood cells through the matrix and minimizes the destruction of cells in the matrix, while portion of the sample that is visible from the test surface does not absorb light at any appreciable point at approximately 700 nm, and c) a ective to indicate the concentration of glucose creating a change in reflectance at 635 on the test surface

Description

REACTIVE PE TEST STRESS FOR PETER PIPING THE 6LUCQSA IN L6 SAN6RE BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a dry test strip for measuring the concentration of an analyte in a biological fluid; more in particular, a test strip that colorimetrically measures the concentration of glucose in whole blood.

Description of the Related Art Many visual test devices have been developed to measure the concentration of certain analytes in biological fluids. These devices have, for example, measured glucose, cholesterol, proteins, ketones, phenylalanine or enzymes in the blood, urine or saliva. Dry-phase reagent strips that incorporate enzyme-based compositions are used extensively in hospitals, clinical laboratories, medical offices and houses to test samples < biological fluids for glucose concentration. In fact, reagent strips have become a daily necessity for many of the several million diabetics < Jel country. Since diabetes can cause dangerous abnormalities in blood chemistry, it can contribute to loss of vision, kidney failure and other medical consequences. To minimize the risk axis these consequences, the current teaching advises people with diabetes to measure their blood glucose level two to seven times a day, depending on the nature and severity of their individual cases. Based on the pattern observed in the glucose levels measured, the patient and the doctor make adjustments together in diet, exercise and insulin consumption to better manage the disease. Clearly, this information should be available to the patient immediately, through < The use of an easy-to-use meter and strip system is quick, inexpensive and accurate. Reagent strips containing an indicator that changes to a different color hue are known, depending on the concentration of glucose in a biological fluid that has been applied to the strip. Although some of these strips use reduction chemicals, they most commonly include oxidizable dye or a dye copula. Some of the strips include an enzyme such as glucose oxidase, which is capable of oxidizing glucose to gluconic acid and hydrogen peroxide. They also contain an oxidizable dye and a substance that has a persyllative activity, which is capable of selectively catalyzing oxidation of the oxidizable dye in the presence of hydrogen peroxide. The patent of E.U.A. No. 4,935,346, expedieja on June 19 < 1990 to R. Phillips et al., describes a meter, a strip and a method for determining the concentration of glucose in a whole blood sample (see also U.S. Patent No. 5,304,468). The method includes simply applying a whole blood sample to a first surface ("sample") of an inert porous matrix that is impregnated with? N reactive. The sample migrates to the opposite "test" surface, by interacting the glucose with the reagent to produce a light-absorbing reaction product. A reading of the reflectance of the test surface indicates the glucose concentration. Reflectance measurements are made at two separate wavelengths to eliminate interference. A time control circuit is activated by an initial decrease in reflectance caused by the wetting of the test surface by the sample that has been passed through the matrix. The patent of E.U.A. No. 5,306,623, issued April 26, 1994 to Kiser et al., Discloses a visual test strip axis of blood glucose which includes applying a whole blood sample containing glucose to one side of the strip and taking the reading from glucose on the opposite side, after the red blood cells have been separated and the sample reacted with a reagent on the strip. It was found that an anisotropic polysulfone membrane was especially useful as a single layer matrix for the ti.

The patent of E.U.A. No. 5,453,360, issued September 26, 1995 to Y.S. Yu describes a colouration of dyes useful in dried reagent strips to "detect analytes, such as glucose, in biological fluids. Copying of dyes consists of hydrazone < 3-Methyl-2-benzothiazolinone and 8-anilino-1-na-talents are used as an indicator in a reaction cascade that produces a strong oxidizing agent such as hydrogen peroxide. An advantage of the copula is that it is soluble in an aqueous solution, but becomes insoluble under oxidizable coupling, thus minimizing the gradual fading of color and providing a stable complete evaporation point. One meter that has been widely accepted for use in self-monitoring of blood glucose is the One Touch Meter < F II, which uses a strip that is described in the patents of E.U.A. Nos. 4,935,346 and 5,304,468, mentioned above. The meter and strip allow the user to measure the glucose concentration in a whole blood sample in a fast, easy and accurate manner. The sample is applied to a surface of the strip in the measurement made on the opposite surface. A portion of the whole blood sample penetrates from the sample surface to the test surface and the color of the blood can be observed from the test surface.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a reagent test strip for use in an apparatus for determining a glucose concentration in a whole blood sample. The apparatus consists of optical means for detecting light intensity at wavelengths (about 635 nm and approximately 700 nm reflected from at least a portion of a matrix disposed near one end of the strip, and such a matrix). consists of: a) a sample receiving surface to receive the whole blood sample and pass a portion of it to a test surface opposite the isma, b) a structure that selectively delays the passage of red blood cells to through the matrix and minimizes the destruction of the cells in the matrix, while any portion of the sample that is visible from the test surface does not absorb light at any appreciable point at approximately 700 n, and c) a reagent to indicate the concentration of glucose creating on the test surface a change in reflectance at approximately 700 nm which is substantially equivalent to that produced by the absorbance of hemoglobin a in blood and a change in reflectance to approximately 635 nrn which is an indicator of glucose concentration. In the present description and the appended claims, reference is made to the pivotal fact that "the sample which is visible from the test surface does not absorb light at any appreciable point at approximately 700 n" means that an absorbance of 700 nrn for the sample, as observed through the test surface, it is less than about 20% of the absorbance of 700 n caused by the reaction of the sample with the reagent. Another embodiment of the present invention also provides a reagent test strip for use in an apparatus for determining a glucose concentration in a whole blood sample. The apparatus consists of optical means for detecting light intensity at wavelengths of approximately 635 nm and approximately 700 nm reflected from at least a portion of a matrix disposed near one end of the strip, and such matrix consists of: a) a sample receiving surface for receiving the whole blood sample and passing a portion of it to a test surface opposite thereto, the test surface has a reflectance at approximately 700 nm which, when the test surface is wetted, undergoes a change that is substantially equivalent to that produced by the absorbance of hemoglobin in the blood, b) a structure that selectively delays the passage of red blood cells through the matrix and minimizes the destruction of cells in the matrix, while any portion of the sample that is visible from the test surface does not absorb light at any appreciable point at approximately 700 n, and c) a reagent to indicate the concentration of glucose will create a change in the reflectance on the test surface at approximately 635 nm. The invention provides a reagent test strip that is suitable for use in a One TouchCR >whole blood glucose meter; Since the structure of the strip selectively delays the passage of red blood cells through the matrix and minimizes e? destruction, glucose determination is less dependent on the hematocrit of the whole blood sample.

BRIEF DESCRIPTION OF THE IBUJO Figure 1 is a perspective view of one embodiment of a test strip of this invention.

DETAILED DESCRIPTION OF THE INVENTION The invention provides a quick and simple method, employing a reliable and easy-to-use device for the determination of whole blood glucose. The method includes applying to a surface (the "sample" surface) "an inert porous matrix a small sample of whole blood, sufficient to saturate the matrix. The matrix is typically present in a reflectance measuring apparatus when the blood is applied. At least a portion of the liquid sample penetrates the matrix, resulting in an initial change in the surface reflectance ("test") opposite. The glucose in the sample reacts with one or more reagents bound to the matrix to form a product that changes the reflectance of the matrix. A reading is then turned on one or more times after the initial change in reflectance to relate to the additional change in reflectance on the test surface or in the matrix, with the glucose concentration in the sample. Figure 1 shows one embodiment of the present invention. A thin hydrophilic matrix reagent pad 11 is positioned at one end of a plastic holder 12 by means of an adhesive 13, which fixes directly and firmly the reagent pad to the holder. The holder, which is optional, provides physical and rigidity to the strip. A hole 14 is present in the plastic holder 12 in the area in which the reagent pad 11 is fixed, so that a sample can be applied through the hole 14 to the sample side of the reagent pad and light be reflected from the other side of the test. A sample of whole blood to be tested is then applied to pad 11. Generally, the sup > The surface of the reagent pad and its area is approximately 10 mM to 100 mm *, especially 10 mMa to 50 mM, which normally provides a volume of which 5-10 μL of sample will be more than saturated. Additional details in reference to the structure of the strip appear in the patents of E.U.A. above mentioned Nos. 4,935,346, ('346) and 5,304,468 (' 468), to be incorporated herein by reference. The method of analysis of this invention is based on a change in absorbency as measured by the diffused reflectance, which depends on the concentration of glucose present in a sample being tested. This change can be determined by measuring the change in reflectance over one or more time intervals. In operation, the test strip is first mounted on an instrument to read the absorbance of light; e.g., color intensity, by reflectance, before application to the sample. Subsequently, a blood sample containing glucose, obtained from finger, for example, is applied to the matrix of the test strip. Preferably, the amount exceeds that necessary to saturate the matrix in the area where the reflectance will be measured (ie, approximately 5-10 μL). After the sample is applied, the measurement time control is automatically started when the fluid enters the matrix and the apparatus detects the resulting change in reflectance of the test surface. The change in reflectance for a predetermined time, as a result of the formation of a reaction product, is subsequently related to the concentration of glucose in the sample. Reflectance is referred to in this description and the appended claims to both the visible wavelength scale as well as the infrared and ultraviolet radiation. A suitable instrument, such as a diffusion reflectance photometer with appropriate software, can be manufactured to automatically read the reflectance in one or more time intervals, calculate the change in reflectance and, using calibration factors, emit the glucose concentration in the blood sample. Details of such an instrument, including the methodology used by the instrument to convert reflectance measurements into blood glucose concentrations, are provided in '346 and' 468. In particular, One Touch <meters; ß commercially available are suitable for use in combination with the reagent strip of the present invention for measuring glucose concentrations in whole blood samples. These meters read the reflectance of the test surface of the strip at approximately 635 nm and approximately 700 nm. The matrix of the present invention is preferably a membrane that effectively separates red blood cells and hemoglobin from a whole blood sample to leave the plasma containing glucose. The separation occurs as the sample moves through the membrane from the sample surface to the test surface. A membrane to achieve that separation may have pores that trap red blood cells, generally pore sizes on the scale of from about 0.1 μm to about 5 μm. Preferably, the membrane is anisotropic, with a scale of pore sizes; most preferably, a wide scale of pore sizes. When the matrix consists of an anisotropic membrane, the sample side is preferably the large pore side. For example, a gradient of sizes (pore size from about 0.1 μm to about 150 μm) may extend across the membrane.On the large pore side, the pore size is preferably in the range from about 30 μm to about 40 μm On the side of the membrane where the pores are smaller (ie the test surface), the void volume is relatively small, and the membrane material is generally quite dense, within a layer that it can typically constitute up to 20% of the thickness of the membrane Within this layer, the pore size is preferably in the range from about 0.1 to about 0.8 μm, with a nominal pore size preferably of about 0.3 μm. of whole blood is applied to the sample side, the sample finds pores smaller and smaller as it penetrates the membrane, eventually, solids such as red blood cells Blood reaches a position in the membrane, usually near the sample surface, where they can no longer penetrate. The membrane not only traps red blood cells near the test surface, but also minimizes the destruction of cells, so that any portion of the sample that is visible from the test surface does not absorb light at any point appreciable at approximately 700 nm. The balance of the sample, which still contains the dissolved glucose, penetrates to the test side. Upon passage through the membrane, the glucose in the sample reacts with the reagent, causing a light-absorbing dye to form near the test side, thereby substantially affecting the reflectance of the test surface. The anieotropic nature of the membrane and / or the use of a separation component (described below) allows relatively rapid flow rates through the membrane even while separation of the solids is taking place. The matrix is a hydrophilic porous membrane to which the reagents can be covalent or non-covalently bound. The matrix allows the flow of an aqueous medium through it. It also allows the binding of protein compositions to the matrix without appreciably and adversely affecting the biological activity of the protein, e.g., the enzymatic activity of an enzyme. To the extent that the proteins will be covalently bound, the matrix will have active sites for covalent binding or can be activated by means known in the art. The composition of the matrix is reflective and has a sufficient thickness to allow the formation of a light absorbing dye in the void volume or on the surface to substantially affect the reflectance from the matrix. The matrix can be of a uniform composition or a coating on a substrate that provides the necessary structure and physical properties, like hydrophilicity. Polysulfones and polyamides (nylons) are examples of suitable matrix materials. Other polymers that have comparable properties can also be used. The polymers can be modified to introduce other functional groups that provide charged structures, so that the surfaces of the matrix can be neutral, positive or negative. A preferred method for preparing the porous material forming the matrix is to cast the polymer without a support center. Such a matrix is, for example, the anisotropic polysulfone membrane available from Memtec, Inc., Tirnoniurn, MD. The terms "matrix" and "membrane" are used interchangeably herein. It is understood that each term is not limited to a single layer and may include, for example, an absorbent layer. A matrix of less than about 500 μm in thickness is usually employed, with about 115 to 155 μm being preferred. A thickness of about 130 to 140 μm is very preferred, particularly when the matrix is nylon or anisotropic polysulfone. The matrix is generally not deformed under wetting, thus retaining its original conformation and size, and has sufficient moisture resistance to allow routine manufacture. The membrane has impregnated in its pores a test reagent which is capable of reacting with glucose to produce a light absorbing reaction product. The membrane can be treated with a reagent by immersing it in a mixture of the components, thus saturating the membrane. The excess reagent can be removed by mechanical means such as, for example, an air knife, a blade, or a glass rod. The membrane is subsequently dried. The reagent tends to concentrate near the small (test) pores side of the membrane. Other methods that are suitable for applying membrane reagent will readily occur to a person skilled in the art. The test reagent consists of a component to convert glucose to hydrogen peroxide and a component to detect hydrogen peroxide. The reagent may additionally and optionally consist of a separating component that causes solids, such as red blood cells, to be trapped in the matrix, effectively removing solids from whole blood. Additional components may also be included as will be described later. Preferred components for converting glucose to hydrogen peroxide include glucose oxidase, an enzyme that is normally obtained from nsperq.illts? Jqer or Penici iurn.The glucose oxidase reacts with glucose and oxygen to produce gluconolactone and p > hydrogen peroxide. The optimum concentration of glucose oxidase depends on the composition (the indicator system, however, the glucose oxidase on the scale of from about 500-10,000 U./rnL, is generally adequate, most preferably from about 700-2000 U. Generally, higher concentrations of glucose oxidase cause the reaction to proceed more quickly and the concentrations lower, less quickly.The optimum concentration can be determined by routine experimentation.The hydrogen peroxide thus produced reacts with The component for detecting hydrogen peroxide, which consists of a peroxidase that selectively catalyzes a reaction between hydrogen peroxide and an indicator.Periodide uses hydrogen peroxide as an oxidant that is capable of removing hydrogen atoms from hydrogen peroxide. Several substrates A suitable peroxidase may contain ferriprotoporphyrin, a red hernin obtained from the Peroxidases obtained from animals, for example from the thyroid gland of animals, are also suitable. Horseradish peroxidase (HRPO) is especially preferred as a constituent of the component for detecting hydrogen peroxide. Hydrogen peroxide, preferably catalyzed by a p > eroxidase, either directly or indirectly reacts to form an indicator dye that reduces the reflectance of 635 nm on the test surface. The reflectance of the test surface is measured at two wavelengths, approximately 635 n and approximately 700 nm. The reflectance measurements are made in a controlled time sequence. The sequence is initiated by the reflectance reduction at 635 nm that results from the arrival of a portion of the sample on the test surface. We qualify this initiation of time control as "reflectance interruption". The reflectance at 700 nm is measured 15 seconds later. By that time, the blood will have saturated the reagent pad, and the interaction of the blood sample containing glucose with the membrane containing reagent will have caused a reduction in reflectance at 700 nm that is subetanially equivalent to the reduction produced by the color of blood that is visible on the test surface. In this way, although any sample that is visible from the test surface does not absorb light at any appreciable point at 700 nm, the meter "detects the reduction in reflection eie 700 nm that it associates with the absorbance by the color of blood and that causes it to subsequently make reflectance measurements at approximately 635 nrn. The glucose concentration in the sample is calculated from the reflectance of 635 nm, using the 700 nm reflectance to calculate a correction factor. Note that whenever the blood absorbs at 635 nm, the 700 nm absorber of blood simulation should do the same. Ideally, the blood simulation material should have the same absorbance ratio at 700 nm at absorbance at 635 nm as does whole blood, but for brevity we refer to the absorbance of the blood simulation material at 700 nrn only . The details of the calculation, including the correction for the reflectance of "blood" at 700 nm, appear in the '346, previously mentioned. The reflectance of the test surface reduced to 700 n that simulates the blood color can be done in four alternative ways. First, the membrane may contain a component that absorbs 700 nm radiation and the test surface may be substantially opaque until it becomes more transparent to 700 nm light when it is wet. The absorbing component at 700 nm can be a non-woven material, for example, that is not visible from the dry test surface. The component can also be a support on which the membrane is cast, a coating on the sample surface of the membrane, or the like. Second, the membrane may include a water soluble dye having light absorbance at 700 nm that is substantially increased when the dye enters the solution. For example, the colorant may initially be in the form of finely divided water-soluble crystals, applied to the membrane as white dispersed solids and provide no substantial absorbance at 700 nm. The aqueous sample dissolves the dye, at which point it dyes and absorbs at 700 n. An example of such a dye is copper phthalocyanine. Third, the interaction between glucose and the reagent in the membrane can result in a chromophore that absorbs light at both 635 nm and 700 nm, thus indicating glucose concentration and, at the same time, simulating the presence of blood. Finally, the interaction of reagent and blood can produce two chromophores, one that absorbs 635 n and the other that absorbs at 700 nm. In addition, since only a reflectance of 700 nm sufficient to stimulate the presence of blood color is needed, preferably only a small amount of the chromophore absorbing 700 nm is present. In the first two cases, in which the absorbance of 700 nm (ie, reduced reflectance) results from another membrane component, the absorbance of 700 nm does not require a chromophore. In each case, however, the reduction in reflectance of 700 nm observed from the test surface 15 seconds after the reflectance interruption must simulate the color of the blood, as described in detail in '346. The magnitude of the reflectance reduction at 635 nm, adjusted as described in '346, at an appropriate time after the initiation of the time control sequence, is a measure of the glucose concentration in the whole blood sample. The dye copulas that are suitable as indicators include 4-aminoantipyrene (AAP) and chromotropic acid; AAP and 8-anilino-lnaphthalene sulfonate (ANS); AAP and N-etii-N- (2-hydroxy-3-sulfopro-yl) -rn-toluidine (TOOS); 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) and ANS; MBTH combined with 3-dimethylaminobenzoic acid and MBTH combined with its forrnaldehyde azine. Although the anisotropic membrane, which is the preferred matrix, filters red blood cells and keeps them out of the test side, the test reagent can optionally also contain a separation component (see, for example, the US patent). No. 5,306,623, to Kiser et al., Mentioned above). The separation component must be capable of producing a relatively clear colorless fluid of fluid containing red blood cells, e.g., whole blood, sequestering red blood cells in the matrix and, preferably, also sequestering any amounts small hemoglobin in free form. Separation components for use in the present invention include but are not limited to polyethylene glycol, ether-poly (dialkyl vinyl / maleic anhydride), polypropylene glycol, polyetheylene sulfonic acid, polyacrylic acid, polyvinyl alcohol and polyvinyl sulfonic acid at a pH of about 4.0. -8.0 Such separation components are present in the matrix in amounts that will vary depending on their charge and molecular weight, with the other components embedded in the matrix, the pH of the matrix and the pore size and the residual moisture of the matrix after drying. . Such p-parameters are easy to determine by a person skilled in the art. For example, when polypropylene glycol is used as the separation component (e.g., PPG-410 from BASF, Uyandotte, MI), it is preferably present at about 2-30% weight to volume (w / v), and most preferably 8-10% w / v. Other separation components can also be used in a concentration of about 2-30% w / v. The comp > Polymeric separation beads can be impregnated or embedded in the matrix or cast in the membrane during fabrication. Some water-soluble salts can also effect blood separation. Among the salts suitable for sep > Plow the components of the blood are citrates, formates and sulfates, as well as certain acids, such as amino acids, citric acid, phytic acid and malic acid. (See, e.g., U.S. Patent No. 3,552,928, issued January 5, 1971 to M.C. Fetter). The separation components are preferably included in the test reagent, because they increase the effectiveness of the membrane in ensuring that no appreciable amount of red blood passes. They insure p > Therefore, the sample that is visible on the surface of the test does not absorb light at any appreciable point at 700 nm. Other components can be embedded in the matrix to improve the coloration and readability of the reagent strips and to preserve the uniformity and integrity of the matrix. For example, the test reagent may include salts and / or pH regulators as auxiliaries in the separation of the dye in the matrix. Such pH regulators may contain, for example, citrate, present in the solution at from about 0.01M to about 1 .OM, and preferably at about 0.1M. Other pH regulators can also be used. Compounds which make the matrix hydrophilic or compounds that can act as stabilizers, such as hydrolysed proteins, can also be employed. Such compounds include but are not limited to for example bovine serum albumin, polypeptides and the low molecular weight protein available as Crotein SPA (CRODA, Inc., New York, N.Y.). Such compounds are used at concentrations of for example about 1 mg / mL to 100 mg / mL approximately. In the case of Crotein, approximately 30 mg / mL are preferred. Other stabilizers can also be included and keep «Jores in the coating for the matrix. For example, ethylenediarinotetraacetic acid (EDTA), diethientriaminepentaacetic acid (DTPA) and related compounds can be employed, for example, at concentrations of about 0.01 mg / mL to about 10 rnG / rnL. Variations may be made to the details presented herein without departing from the scope and spirit of the present invention.

Claims

00 NQVEPAP PE THE INVENTION CLAIMS

1. - A reagent test strip for use in an apparatus for determining a glucose concentration in a whole blood sample. The apparatus consists of optical means for detecting light intensity at wavelengths of approximately 635 nm and approximately 700 nm reflected from at least a portion of a matrix disposed near one end of the strip, and such a matrix consists of: a) a sample receiving surface for receiving the whole blood sample and passing a portion of it to a test surface opposite thereto, the test surface has a reflectance at approximately 700 nm which, when the test surface is moistens, suffers a change that is substantially equivalent to that produced by the absorbance of hemoglobin in the blood; b) a structure that selectively delays the passage of red blood cells through the matrix and minimizes the destruction of cells in the matrix, while any portion of the sample that is visible from the test surface does not absorb light at no appreciable point at about 700 nm, and c) a reagent to indicate the glucose concentration by creating a change in reflectance at 635 nm on the test surface.

2. The strip according to claim 1, wherein the matrix consists of a membrane that has pores that trap the red blood cells < he shows the whole blood.

3. The strip according to claim 1, wherein the matrix consists of an anisotropic membrane.

4. The strip according to claim 3, wherein the membrane has pores that are larger near the sample receiving surface and smaller near the test surface.

5. The strip according to claim 1, wherein the matrix consists of a polyamide membrane.

6. The strip according to claim 1, wherein the matrix has an interior region that absorbs light at 700 nm and which is visible from the test surface only when the surface is wet.

7. The strip according to claim 6, wherein the inner region of the matrix consists of a nonwoven material having a substantial absorbance at 700 nm.

8. The strip according to claim 1, wherein the matrix additionally consists of a water-soluble dye having a light absorption at 700 nm which is substantially increased when the dye is converted into a solution.

9. The strip according to claim 8, wherein the dye consists of copper phthalocyanine.

10. The strip according to claim 1, wherein the reagent consists of a dye precursor which forms a chromophore indicating the concentration of glucose, and the chromophore absorbs light at 635 nm. II.- The strip according to claim .1.0, in which the dye precursor of the reagent is selected from the group q? E consists of hydrazone hydrochloride «je 3-rnetii-2-benzothiazolinone (MBTH) with 3- dimethylaminobenzoic acid (DMAB); MBTH with 8-anilino-l -naphthalenesulphonate (ANS); MBTH with its formaldehyde azine and combinations thereof. 12. The strip according to claim 11, wherein the dye precursor consists of MBTH and ANS. 13. The strip according to claim 11, wherein the dye precursor consists of MBTH combined with DMAB.