Evaluation of CK-MB isoform analysis for early diagnosis of myocardial infarction

Measurement of CK-MB and its isoforms by high-voltage electrophoresis has been proposed as a sensitive test for early detection of myocardial infarction (MI). We performed a prospective study of this test in 231 patients presenting to the Emergency Department with symptoms consistent with ischemic chest pain. Blood specimens were obtained at 0, 1, and 3 h following presentation, and plasma was immediately frozen and analyzed within 1 week by high-voltage electrophoresis for total CK-MB and isoforms. The test was considered positive whenever total CK-MB was elevated (>6 U/L) or the cardiac isoform MB2 was relatively increased (MB2 > 2 U/L and MB2/MB1 > 1.7). This test had a sensitivity of 68% overall and 55% for specimens collected within 3 h of symptom onset. It was positive within 3 h of presentation in 36/39 (92%) of patients with confirmed MI. Specificity was 92% overall and did not vary with time after symptoms. The CK-MB alone, at the cutoff of 6 U/L, had lower sensitivity overall (56%; p = 0.01) and within 3 h of onset (39%; p = 0.03), and higher specificity overall (98%; p < 0.001). Lowering the cutoff for CK-MB alone to match the sensitivity of the isoform test caused a greater loss of specificity. It is concluded that analysis of CK-MB by high-voltage electrophoresis is an effective method for rapid diagnosis of MI, with the isoform analysis enhancing early sensitivity.