NADH preparations as they affect reliability of serum lactate dehydrogenase determinations.

We observed large variations in the activity of lactate dehydrogenase (EC 1.1.1.27) in serum when random lots of NADH were used in its determination. The imprecision, caused by variable amounts of lactate dehydrogenase inhibitor in the NADH preparations, was greatly lessened by using only a single lot of NADH; reliability was improved further by selecting an NADH preparation having a negligible amount of inhibitor. A method is described for assessing the quality of commercial NADH preparations, so as to have a satisfactory reagent and to maintain continuously comparable results over an extended period of time. The results have possible relevance for proposed governmental regulation of commercial kits and reagents used for clinical laboratory testing, and for improved intraand inter-laboratory comparability of lactate dehydrogenase determinations.