Serum Lactic Dehydrogenase Activity: an Analytical Assessment of Current Assays.

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 serumadded,anda reproducibilityof 1.3%. The reagent mixtore can be stored frozen for 6 monthsand in the lyophilized state indefinitely. Poor separationof normal from abnormal serum LDH activities results from the nonlinearcalibrationcurves anddecreasingreaction rates of the “backward” spectrophotometric assay. The dinitrophenylhydrazinecolorimetric assay is deemed unsuitable for clinical use because largevariationsin activity result from minor variations in the reagent calibration curve, and because colorimetric activities are inaccurate as compared to activities obtained with the “forward” spectrophotometricassay.