Ldh isoenzymes by urea inhibition and substrate (lactate) modification: a clinical evaluation

Summary 1. Differentiation of the anodic (LDH 1 and LDH 2 ) and cathodic (LDH 5 ) isoenzymes of lactate dehydrogenase in human sera can be accomplished by combining variable substrate (lactate or pyruvate) concentrations with specific inhibitors such as urea. 2. One technic useful in the clinical laboratory is based upon a colorimetric assay for total LDH activity in which the oxidation of lactate is coupled to the reduction of a tetrazolium salt. The ratios of activity in 2M lactate to the activity in 0.02 lactate—2M urea yields an index of the predominant LDH isoenzymes in sera. 3. Clear-cut distinction between myocardial and hepatic isoenzyme predominance was achieved in the majority of 98 patients studied, but the isoenzyme index can be misleading and non-confirmatory in patients who have liberation of LDH isoenzymes from several organ sources. 4. The LDH isoenzyme index is considered to be an additional and informative test only and cannot serve as a diagnostic estimation.