Usefulness of various lactate dehydrogenase isoenzyme 1 profiles after myocardial infarction.

Isoenzyme profiles of lactate dehydrogenase (percent LD-1 of total LD, LD-1/LD-2 ratio, and absolute LD-1) have all been studied as late markers for myocardial infarction. It is known, however, that elevations of LD-5 frequently occur in this period as a result of liver congestion. Elevations of LD-5 may also occur as a result of complicating conditions. Such elevations could result in a reduced percent LD-1 of total LD, giving rise to false negatives. Receiver operating characteristic (ROC) curves were constructed for LD-1, LD-1/LD-2 and percent LD-1 of total LD from 285 specimens (124 patients) with suspected myocardial infarction. There was little difference in overall diagnostic power among the three assays. Using cutoffs determined from the ROC curves, 6 patients (18 specimens) were evaluated who appeared to be in the late period or who exhibited complicating conditions which could increase LD-5. In 14/18 specimens, increases in LD-5 resulted in false negatives by percent LD-1 of total LD. Only 5/18 specimens were false negatives by LD-1 or LD-1/LD-2. It is concluded that the percent LD-1 of total LD was affected by an increase in LD-5, and caution is recommended when using it.