Serum creatine kinase (CK) activity after M-subunit inhibition in patients with atypical CK isoenzymes--a comparison to findings in acute myocardial infarction.

The serum levels of total creatine kinase (CK), and of CK-B, as estimated by the enzymatic anti-M-subunit immunoinhibition method, were studied in 14 patients with CK-B elevation associated with advanced malignant disease, in nine subjects with electrophoretically verified, immunoglobulin-bound CK-BB (macro CK) and in 28 patients with acute myocardial infarction (AMI). The range of CK-B activity was similar in all three groups. In AMI, the ratio CK-B/total CK at peak CK-B was less than 13% (mean 7%). In both the groups with atypical CK, the ratios varied from about 30% to nearly 100%, with a mean amounting to 2/3 of total CK activity. CK-B elevations in patients with untreated malignant tumours tended to increase with time, but occasionally remained fairly constant for months, like those in subjects with macro CK. Complementary CK isoenzyme separation, e.g. by electrophoresis, is needed to differentiate conditions with atypical CK activity, detected by routine use of CK-B determinations in the diagnosis of AMI.