Stable colored reference standard for enzymatic determinations

Abstract

A stable, colored reference standard suitable for use in diagnostic determinations involving enzymatic reactions in which colorless 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride is reduced to the red-colored 1-(p-iodophenyl)-5-(p-nitrophenyl)-3-phenylformazan, is described. The reference standard of this invention is an aqueous solution containing, in specified amounts, 1-(p-iodophenyl)-5-(p-nitrophenyl)-3-phenylformazan; serum albumin; an N,N'-dimethylformamide or a dimethylsulfoxide solvent; and isopropyl alcohol. An inert bulking agent may be added to the aqueous colored standard and the solution obtained may then be lyophilized. The lyophilized form of the colored reference standard is stable for at least 3 months at 4.degree. C. and 24.degree. C. and can be easily reconstituted with water or with dilute aqueous hydrochloric acid solutions. The novel colored reference standard solution has an absorbence maximum of 500 nanometers. The colored reference standard of this invention may be used in the determination of serum lactate dehydrogenase, creatine phosphokinase, glucose-6-phosphate dehydrogenase, adenosine phosphate, glucose, glucose-6-phosphate, 6-phosphogluconate and the like.