A diagnosis of myocardial infarction (MI) is usually established by the evaluation of clinical symptoms, electrocardiographic changes, and serum enzyme levels, specifically creatine phosphokinase, subunit MB (CK-MB), by electrophoresis. A total of 215 patients were evaluated in this study. One hundred two of them were admitted to the coronary care unit and 113 to the emergency room, where they were screened for possible MIs. The radioimmunoassay (RIA) used in this study determines levels of the CK-MB isoenzyme by detecting the B monomer, which also has 100% cross-reactivity with the CK-BB isoenzyme. The intra-assay coefficients of variability (CVs) for 30 samples were 22% (x̄ = 7.0 ng/ml) and 11% (x̄ = 47.3 ng/ml), and the interassay CVs for 30 samples were 17% (x̄ = 7.1 ng/ml) and 9.2% (x̄ = 49.3 ng/ml). Of the 215 patients evaluated, 21 had MI by the criteria in the study. The diagnostic sensitivity, specificity, and accuracy were 100.0%, 92.8%, and 93.5%, respectively. These values increased to 100.0%, 96.9%, and 97.2% when only coronary care unit patients were considered. The CK-MB RIA was found to be a reliable replacement for electrophoresis, but it was nonspecific in some patients.
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