Patent ID: 7432072

Claim:
A method of measuring an oxidizing substance derived from an analyte in a sample, comprising: causing a reaction between the oxidizing substance derived from the analyte and N-(carboxymethylaminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt as a substrate that develops color as a result of the reaction, the reaction being caused by an oxidoreductase in the presence of a tetrazolium compound and sodium azide, the N-(carboxymethylaminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt, the tetrazolium compound and the sodium azide being present in an aqueous solution with a surfactant; and determining an amount of the oxidizing substance by measuring the color developed by the substrate, the method farther comprising, before the step of causing a reaction, determining an amount of the tetrazolium compound, the sodium azide, and the surfactant, and a pH of the solution, that will reduce erroneous color formation of the N-(carboxymethylaminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt as a result of the reaction, and mixing the tetrazolium compound and the sodium azide in the determined amounts with the N-(carboxymethylaminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt in the presence of the determined amount of the surfactant in an aqueous solvent, wherein the determined amounts are selected to provide concentrations in the mixture in the ranges of 0.01 to 1 mmol of the tetrazolium compound, 0.003 to 0.5 mmol of the sodium azide, and 0.006 to 0.4 mmol of the surfactant, per μmol of the N-(carboxymethylaminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt, and the pH of the mixture is in a range of 6 to 9.