Increased creatine kinase (CK) MB isoenzyme in patients with "normal" total CK activity suspected of acute myocardial infarction.

Many laboratories screen patients suspected of acute myocardial infarction (AMI) with a test for total creatine kinase (CK) and perform testing for CK-MB isoenzyme only in patients with elevated total CK. To find out whether this practice could result in missing patients with AMI who can have "normal" (within the reference interval) total CK with increased CK-MB (greater than or equal to 5%), we prospectively and sequentially monitored CK and CK-MB in patients admitted for suspected AMI. We found that 12.5% of patients with the final diagnosis of AMI had initially low total CK and high CK-MB (as determined by electrophoresis), but the majority of these patients showed subsequently increased total CK above the reference value. It is suggested that the presence of CK-MB in patients with low total CK does not represent a laboratory error and most of these patients have AMI. Total CK assay could be abandoned in favor of CK-MB testing in patients suspected of AMI.