Methods and compositions are provided for determining ADP in the presence of ATP. These comprise including among the assay reagents at least one of the correcting components creatine phosphokinase and phosphocreatine, pyruvate kinase and phosphoenolpyruvate, peroxidase and a non-interfering peroxidase substrate, and catalase. One aspect of the method employs formation of hydrogen peroxide from the ADP by pyruvate kinase, phosphoenolpyruvate and pyruvate oxidase. The hydrogen peroxide is then determined. A combined reagent having all of the reagents may optionally include a peroxidase when the hydrogen peroxide is to be enzymatically determined. A peroxidase substrate is added to the sample in conjunction with the peroxidase substrate reagent, the mixture incubated and depending on whether the peroxidase substrate is a fluorescer or chemiluminescer, the mixture may be illuminated with excitation light and the emitted light determined as a measure of the ADP in the sample.