Abstract: A test indicator determines concentration, in a biological fluid, of a metabolic acid, such as beta-hydroxybutyrate or lactic acid. The indicator includes a test surface containing the dried residue resulting from impregnation of the surface with components including a tetrazolium salt, nicotinamide adenine dinucleotide (NAD), an electron carrier, and the enzyme that is a dehydrogenase for the specific metabolic acid being assayed. The indicator also includes a physically distinct provision for buffering the reaction. In accordance with the concentration of the metabolic acid being assayed, the indicator provides a predetermined color or intensity which can be observed either with the naked eye or with instrumentation such as a reflectance meter.

Abstract: There is provided a procedure for determining the concentration of a metabolic acid in a biological fluid. The method involves use of a tetrazolium salt, a pyridine nucleotide, an electron carrier, and an enzyme that is a dehydrogenase for the specific acid being assayed. These components are used to form an assay mixture; thereafter a quantity of the fluid to be assayed is combined with the assay mixture, so that there may commence a reaction in which the tetrazolium salt is changed to a formazan in an amount that is indicative of the concentration of the specific acid. In a preferred embodiment, the procedure permits assay of the concentration of beta-hydroxybutyrate, and the assay mixture includes components in the relative proportions of 0.8 micromoles 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride (INT), 0.32 micromoles phenazine methosulfate (PMS), 1.5 micromoles nicotinamide adenine dinucleotide (NAD), and 0.