Multilayer analytical element for assaying fructosamine

Abstract: A multilayer analytical element for assaying fructosamine having a liquid-impermeable support, a buffer-containing layer which contains a buffer having pH of 8 to 12 and is formed on the support, a tetrazolium salt-containing layer which is formed on the buffer-containing layer and optionally an intermediate layer interposed between the buffer-containing layer and the tetrazolium salt-containing layer to prevent contact of these two layers, which element assays fructosamine in a short time with good accuracy.

Description: The present invention relates to a multilayer analytical element for assaying fructosamine in a body fluid, in particular, serum. For the measurement of fructosamine in the serum, Johnson and Baker proposed a method comprising measuring a reducing power of fructosamine with Nitrotetrazolium Blue (NTB) under an alkaline condition (see Clinica Chimica Acta, 127 (1982) 87-95, Japanese Patent Publication No. 13062/ 1989, EP-A-085 263 and U.S. Patent Nos. 4,642,295 and 4,645,742). In this method, the serum is preincubated for 10 minutes to remove influence of reducing substances in the serum other than fructosamine by reacting such reducing substances with NTB to decompose them, and then an amount of consumed NTB in the following 5 minutes period (namely from 10 minutes to 15 minutes) is measured since the amount of NTB which is consumed in this 5 minutes period is proportional with the amount of fructosamine in the serum. Japanese Patent Kokai Publication No. 182567/1988 and EP-A-215 170 disclose an end point assay in which interfering substances present in a sample are removed by treating them with a strong base, an oxidizing agent or an enzyme or by desalting, pH of the sample is adjusted in a range between 10 and 14, a color reagent (a tetrazolium salt) is added and then a developed color is measured. Japanese Patent Kokai Publication No. 15168/1988 and EP-A-250 991 disclose a method comprising pretreating a sample at neutral pH to remove reducing substances and/or turbid materials, adjusting pH in a range between 10 and 12 and adding a color reagent. Japanese Patent Kokai Publication No. 304999/1988 and EP-A-291 060 disclose a method comprising adding a buffer solution having pH of 9 to 12, a color reagent, uricase and at least one detergent (surfactant) and, after 5 minutes at earliest, chemical kinetically measuring change of an absorbance with time in a temperature range between 20°C and 40°C. The method of Johnson and Baker is a so-called wet chemistry method and adjusts a temperature at 37°C under the alkaline conditions and then measures change of an absorbance in a time range between 10 minutes and 15 minutes. The method of Japanese Patent Kokai Publication No. 182567/1988 and EP-A-215 170 is a two-step method, which is troublesome and time-consuming. In the method of Japanese Patent Kokai Publication No. 304999/1988 and EP-A-291 060, the preincubation requires at least 5 minutes before the reducing substances are removed and turbidity is clarified. EP-A-0 473 101 published on March 04, 1992 but claiming the priority date of August 29, 1990, thus only relevant under Art. 54(3) EPC, discloses an integral multilayer analytical element having a tetrazolium salt-containing layer, an alkaline buffer layer and an intermediate layer interposed between the two aforementioned once. It is stated that the device can be used for the detection of fructosamine. In addition, all the above prior art methods belong to the wet chemistry and should measure the change of absorbance using a reaction cell. In general, formazan, which is generated through reduction of a tetrazolium salt to be used in a color developing reaction with fructosamine, firmly sticks to the reaction cell or a flow cell and increases a background, which is a cause of error. When fructosamine is measured with an automatic analyzer which is widely used in many facilities, generated formazan deposits in a reaction tube, a transportation tube or the flow cell to contaminate the automatic analyzer, so that other components cannot be measured. Since it is difficult to remove deposited formazan, various devices or parts should be immersed in a strong acid or a surfactant overnight. Reagents for measuring fructosamine are strongly alkaline (pH of 10 or larger) and waste liquids therefrom pollute the environment. In contrast to the wet chemistry, a so-called dry chemistry method is known. In the dry chemistry method, since one drop (several µl) of the sample is used for the reaction, the sample is dried when the reaction continues more than 10 minutes and accuracy in measurement is deteriorated. When the measuring time is shortened to 10 minutes or less, the measurement is influenced by the reducing substances in the body fluid, so that accurate data are not obtained. One object of the present invention is to provide an analytical element for measuring fructosamine in a body fluid by a dry chemistry technique. Another object of the present invention is to provide an analytical element for easily measuring fructosamine in a body fluid in a short measuring time. According to the present invention, there is provided a multilayer analytical element for assaying fructosamine comprising a liquid-impermeable support, a buffer-containing layer which contains uricase and a buffer having pH of 8 to 12 and is formed on said support, a tetrazolium salt-containing layer which is formed on said buffer-containing layer and optionally an intermediate layer interposed between said buffer-containing layer and said tetrazolium salt-containing layer to prevent contact of said two layers. Fig. 1 is a graph showing change of a K/S value with time for the sample elements of Example 1 (broken lines) and Example 2 (solid lines). The liquid-impermeable support used in the present invention may be made of any liquid-impermeable material such as a plastic film, paper or a metal foil. Among them, the plastic film is preferred. The buffer may be any one of conventional buffers having a buffering capacity in a pH range between 8 and 12, preferably between 8.5 and 10.5. In particular, a carbonate buffer is preferred. The tetrazolium salt to be used in the present invention may be any one of conventional tetrazolium salts that will react with fructosamine to develop a color. Specific examples of the tetrazolium salt are: 3,3'-[3,3'-dimethoxy-(1,1'-biphenyl)-4,4'-diyl]-bis[2-(p-nitrophenyl)-5-phenyl-2H-tetrazolium chloride] (NTB), 3,3'-[3,3'-dimethoxy-(1,1'-biphenyl)-4,4'-diyl]-bis[2,5-bis(p-nitrophenyl)-2H-tetrazolium chloride] (TNTB), 3,3'-(1,1'-biphneyl-4,4'-diyl)-bis(2,5-diphenyl-2H-tetrazolium chloride] (NeoTB), 3-(p-iodophenyl)-2-(p-nitrophenyl)-5-phenyl-2H-tetrazolium chloride (INT), 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT), and 2,5-diphenyl-3-(1-naphthyl)-2H-tetrazolium chloride (TV). The buffer-containing layer of the multilayer analytical element of the present invention contains uricase. Since uricase decomposes uric acid which is one of the reducing substances in the body liquid in a short time, it will suppress influence of uric acid during the assay of fructosamine and shorten the assay time. The multilayer analytical element of the present invention can be prepared by successively laminating the buffer-containing layer, the optional intermediate layer and the tetrazolium salt-containing layer on the liquid-impermeable support. For example, the alkaline buffer-containing layer is formed on the support by dissolving the buffer and a binder in a suitable solvent, applying a resulting solution on the support, and drying it. Preferably, the binder is a hydrophilic polymer such as polyvinylpyrrolidone (PVP), hydroxypropylcellulose (HPC), methylcellulose (MC), polyacrylamide, gelatin, etc. When the hydrophilic polymer is used, the solvent is an aqueous solvent, in particular, water. When the intermediate layer is provided, it is formed by dissolving a binder resin in a solvent in which neither the buffer nor the tetrazolium salt is dissolved, applying a resulting solution on the formed buffer-containing layer and drying it to form a binder resin film. As the solvent, isopropanol, acetone, chloroform, methyl chloride and toluene are preferred. As the binder resin, HPC, PVP and the like are used. The tetrazolium salt-containing layer is formed by impregnating a solution of the salt in a porous matrix such as a filter paper, woven or knit fabric or a membrane filter and drying it. Then, the porous matrix containing the impregnated tetrazolium salt is laminated on the buffer-containing layer or the intermediate layer. A solvent in which a specific tetrazolium salt is dissolved is well known. To improve the solubility of the tetrazolium salt in the solvent, a surfactant may be added to the solvent. A thickness of each of the buffer-containing layer, the intermediate layer and the tetrazolium salt-containing layer is not critical. Preferably, the buffer-containing layer has a wet thickness is 250 µm or less, e.g. about 200 µm, the intermediate layer has a wet thickness of 200 µm or less, e.g. about 150 µm, and the tetrazolium salt-containing layer has a thickness of 300 µm or less, e.g. about 250 µm. The laminate of the buffer-containing layer, the optional intermediate layer and the tetrazolium salt-containing layer is cut to a suitable size, for example, 5 mm x 7 mm, fixed to a base film with a double coated tape or an adhesive and used in the assay. The assay of fructosamine is carried out by placing a drop of a sample on the multilayer analytical element of the present invention and measuring the developed color by a conventional method. A specific assay manner will be explained in below described Examples. The present invention will be illustrated by following Examples. Example 1 A bufter-containing layer was formed by mixing the following components, applying the resulting mixure on an opaque polyethylene terephthalate film at a wet thickness of 200 µm and drying it at 40°C for 30 minutes. Sodium carbonate 3.18 g Sodium hydrogencarbonate 0.84 g Purified water 86 g PVP K-90 10 g A tetrazolium salt-containing layer was formed by mixing the following components, impregnating a cotton or polyester fabric with the resulting mixture and drying it at 40°C for 30minutes. NTB 700 mg Methanol 10 g Purified water 14.3 g A mixture of HPC (2 g) and isopropanol (98 g) was coated as an intermediate layer on the buffer-containing layer and then the tetrazolium salt-containing layer was laminated thereon and dried at 40°C for 30 minutes. The produced laminated was cut to a size of 5 mm x 7 mm and fixed at one end of a polyethylene terephthalate strip (5 mm x 80 mm) with a double coated tape to obtain a sample element. Linearity A human serum containing fructosamine at a high concentration was stepwise diluted with purified water. Each 6 µl of the diluted serum was spotted on the sample element and incubated at 37°C. Then, a reflectance from 5 to 7 minutes from the start of incubation was monitored. The measured reflectance was converted to a K/S value according to the Kubelka-Munk equation, and its difference (ΔK/S) was calculated. Example 2 A buffer-containing layer was formed by mixing the following components, applying the resulting mixture on an opaque polyethylene terephthalate film at a wet thickness of 200 µm and drying it at 40°C for 30 minutes: Sodium carbonate 0.318 g Sodium hydrogencarbonate 0.084 g Purified water 8.6 g PVP K-90 1.0 g Uricase 4000 Units A tetrazolium salt-containing layer was formed by mixing the following components, impregnating polyester fabric with the resulting mixture and drying it at 40°C for 30minutes: NTB 700 mg Methanol 10 g Purified water 14.3 g A mixture of HPC (2 g) and isopropanol (98 g) was coated as an intermediate layer on the buffer-containing layer and then the tetrazolium salt-containing layer was laminated thereon and dried at 40°C for 30 minutes. The produced laminated was cut to a size of 5 mm x 7 mm and fixed at one end of a polyethylene terephthalate strip (5 mm x 80 mm) with a double coated tape to obtain a sample element. Each 6 µl of a human serum and a human serum containing about 20 mg/dl of uric acid was spotted on each of the analytical elements produced in Examples 1 and 4, and Δ K/S was calculated in the same manner as in Example 1 except the measuring time period. The results are shown in Table 4 and Fig. 4. Example 4 Example 1 ΔK/S between 4 and 6 min. ΔK/S between 5 and 7 min. ΔK/S between 4 and 6 min. ΔK/S between 5 and 7 min. Serum 0.148 0.147 0.158 0.149 Serum + Uric acid 0.150 0.148 0.169 0.152 In Example 4, when the assay time is shifted to the range between 4 minutes and 6 minutes, fructosamine can be assayed without suffering from the influence of uric acid.

Claims: A multilayer analytical element for assaying fructosamine comprising a liquid-impermeable support, a buffer-containing layer which contains uricase and a buffer having pH of 8 to 12 and is formed on said support and a tetrazolium salt-containing layer which is formed on said buffer-containing layer. The multilayer analytical element according to claim 1, which further comprises an intermediate layer interposed between said buffer-containing layer and said tetrazolium salt-containing layer to prevent contact of said two layers. The multilayer analytical element according to claim 1, wherein a tetrazolium salt containing in said tetrazolium salt-containing layer is at least one selected from the group consisting of 3,3'-[3,3'-dimethoxy-(1,1'-biphenyl)-4,4'-diyl]-bis[2-(p-nitrophenyl)-5-phenyl-2H-tetrazolium chloride], 3,3'-[3,3'-dimethoxy-(1,1'-biphenyl)-4,4'-diyl]-bis[2,5-bis(p-nitrophenyl)-2H-tetrazolium chloride], 3,3'-(1,1'-biphneyl-4,4'-diyl)-bis(2,5-diphenyl-2H-tetrazolium chloride], 3-(p-iodophenyl)-2-(p-nitrophenyl)-5-phenyl-2H-tetrazolium chloride, 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide and 2,5-diphenyl-3-(1-naphthyl)-2H-tetrazolium chloride. The multilayer analytical element according to claim 1, a buffer has pH of 8.5 to 10.5.

Inventors: SAKAMOTO HISASHI, SAKAMOTO, HISASHI
Applicants: KYOTO DAIICHI KAGAKU KK, KYOTO DAIICHI KAGAKU CO., LTD.
Application year: 1991
Main classification: G01N33
IPC codes: C12Q1, G01N33