Serum Lactic Dehydrogenase Activity: An Analytical Assessment of Current Assays

Abstract

Abstract Serum LDH activities assayed spectrophotometrically by the "forward" (lactate + DPN to pyruvate + DPNH + H) method have linear reaction rates, activity proportional to the amount of serum added, and a reproducibility of 1.3%. The reagent mixture can be stored frozen for 6 months and in the lyophilized state indefinitely. Poor separation of normal from abnormal serum LDH activities results from the nonlinear calibration curves and decreasing reaction rates of the "backward" spectro-photometric assay. The dinitrophenylhydrazine colorimetric assay is deemed unsuitable for clinical use because large variations in activity result from minor variations in the reagent calibration curve, and because colorimetric activities are inaccurate as compared to activities obtained with the "forward" spectrophotometric assay.