Abstract: This invention relates to a reagent for enzymatic determination of an analyte concentration in a patient wherein the degree of oxidation of a coenzyme is measured, characterized in that said reagent is stabilized against oxidation by a coenzyme reduction system comprising an enzyme and substrate pair selected so as to enable continuous regeneration of said coenzyme throughout storage of said reagent. Also disclosed is an improvement in an enzymatic method of determination of an analyte concentration in a sample body fluid wherein the degree of oxidation of a coenzyme is measured, the improvement comprising stabilizing a reagent comprising said coenzyme against oxidation by a coenzyme reduction system comprising an enzyme and substrate pair selected so as to enable continuous regeneration of said coenzyme throughout storage of said reagent. Also disclosed are reagents for the determination of aspartate aminotransferase, alanine aminotransferase, ammonia and urea.