The prognostic significance of immunoradiometric CK-MB assay (IRMA) diagnosis of myocardial infarction in patients with low total CK and elevated MB isoenzymes

Abstract

Patients with suspected myocardial infarction present a diagnostic problem when they have normal total serum creatine kinase (CK) and an elevated myocardial fraction of this enzyme (CK-MB). We studied 40 patients with normal total serum CK and elevated CK-MB (by standard electrophoretic technique), using an immunoradiometric assay (IRMA) for CK-MB. Diagnosis based on IRMA results of serum samples collected prospectively was compared with diagnosis based on application of strict diagnostic criteria with and clinical diagnosis of the responsible physician(s) by chart review. Diagnostic agreement of all three methods and clinical outcome (mean follow-up 6 months after discharge) were analyzed and compared with two control groups: “rule in” group (17 patients) with elevated total CK, CK-MB, and abnormal electrocardiogram (ECG); and “rule out” group (18 patients) with normal total CK, CK-MB, and ECG. All three diagnostic methods (1) clinical discharge diagnosis, (2) chart review diagnosis by application of strict diagnostic criteria, and (3) IRMA diagnosis, agreed completely for both control groups. Follow-up control group outcomes were in concert with expected outcomes for these groups. Diagnostic differences between methods 1 and 2, and 1 and 3 were statistically significant for the study group. Furthermore, only diagnosis based on the IRMA showed a predictive capability for outcome in this study group when compared with the other methods. Follow-up revealed a similar incidence of cardiac events in the study group (56%) and in the “rule in” control group (60%), but not in the “rule out” group (7%). We conclude that a high percentage (>77%) of our patients with suspected myocardial infarction and normal total CK with elevated CK-MB did suffer myocardial damage and had a course similar to that observed in patients with clearly defined ECG and enzyme criteria for the diagnosis of myocardial infarction. Moreover, the IRMA for CK-MB accurately predicted outcomes for these patients. This method of CK-MB quantifica ion may offer advantages over the standard gel electrophoretic technique of CK-MB determination.