Clinical and analytical evaluation of kits for measurement of creatine kinase isoenzyme MB.

We studied the analytical and clinical performance of six methods for creatine kinase (EC 2.7.3.2) isoenzyme MB (CK-MB): three immunoassays (Behring, Hybritech, and International Immunoassay Labs); one immunoinhibition assay (Roche); one immunoinhibition/column method (Du Pont); and one electrophoretic method (Beckman). Between-day precision for all kits was poor at the upper reference limit. All methods gave results linearly related to CK-MB concentration and all were free from CK-MM, CK-BB, and adenylate kinase interference. Only the Du Pont method was adversely affected by atypical isoenzymes. For diagnosis of acute myocardial infarction in a coronary care population (n = 40; prevalence = 45%), all methods were approximately 95% efficient, when appropriate reference criteria were used. Some manufacturers fail to provide data for an appropriate (acutely ill, non-infarct) reference population; decreased diagnostic specificity may result from use of reference ranges based on results for healthy subjects. Expression of CK-MB as a percent of total CK degrades efficiency unless total CK is markedly increased.