Cold lability of lactate dehydrogenase isoenzymes and the effective preparation of reference material for clinical laboratory use

To evaluate the storage of samples and enzyme reference materials, and to improve the commutability for inter‐laboratory surveillance of activity values of lactate dehydrogenase (LD; EC 1.1.1.27) in clinical laboratory medicine and in animal veterinary medicine, we studied the electrophoretic patterns and cold lability of LD isoenzymes from tissue sources of some common vertebrate species and also from human serum sources. Among many isoenzymes from these sources, only rat LD fractions showed similar electrophoretic patterns to those of human sera, and rat LD‐1 fraction was relatively cold‐stable. Total LD and isoenzyme LD‐1 activities in routine laboratory samples and quality‐control sera were measured using eight kinds of commercially available LD assay kit, including lactate and pyruvate substrate systems. Coefficients of variation between these assay kits were markedly reduced when the activities were calculated using the partially purified rat LD‐1 fraction as an enzyme reference material, compared with the activities calculated using the factor indicated in each assay kit. In the regression analysis, the intercept and slope were calculated for the regression equations obtained from 12 pairs of these assay kits. The values obtained from a small amount of human serum and control serum samples were within the 95% confidence regions of those from larger amounts of human serum samples by using the present rat LD‐1 standard for measuring LD‐1 activities with lactate substrate. It was evident that a cold‐stable and homogeneous LD isoenzyme as an enzyme reference material might contribute to accurate measurement of activities in heterogeneous samples for inter‐laboratory quality‐assurance surveys in both human and animal clinical laboratory use.