Diagnosis of acute myocardial infarction by detection of circulating cardiac myosin light chains

A radioimmunoassay for human cardiac myosin light chains (CM-LC) was developed and evaluated as a selective diagnostic test for acute myocardial infarction (AMI). The assay had a sensitivity of 1.0 ng/ml (±2 standard deviations) in serum. Eighty-three patients with confirmed AMI all showed an elevated plasma concentration of CM-LC at some time during the course of their illness. Of 9 patients from whom early blood samples were obtained, 7 had diagnostic concentrations within 6 hours from the onset of chest pain. Only 2 had an elevated total creatine kinase level at this time. CM-LC concentrations peaked on days 2 to 4, but remained elevated in patients with large AMIs for more than 1 week. In preinfarction syndrome, 8 of 15 patients had elevated CM-LC levels at least once. Of 15 patients with stable angina pectoris, only 1 patient, who had congestive heart failure, showed elevated light chain levels. CM-LC levels were not detectable by this method in the sera of healthy persons (n = 72), patients with recent intramuscular injection (n = 3), or those with a variety of systemic illnesses (n = 14). In initial studies using an antiserum having 25% cross-reactivity between cardiac and skeletal muscle myosin light chains, 3 patients who had extensive skeletal muscle damage appeared to have elevated concentrations. Patients with this finding have not yet been examined with a more specific antiserum (8% cross-reactivity). The duration of CM-LC elevation in the circulation may be related to the extent of myocardial damage. This method may therefore provide a specific and sensitive diagnosis of early as well as late myocardial necrosis.