Sensitive and specific radioimmunoassay for creatine kinase BB isoenzyme in serum, with use of an autoantibody.

We have developed an equilibrium radioimmunoassay for creatine kinase BB (CK-BB) isoenzyme in serum. We used as antibody a CK-BB binding immunoglobulin G isolated from serum of a patient with macro CK-BB. Free and antibody-bound tracer were separated by anion-exchange chromatography. The affinity constant, Keq, for the binding of CK-BB to immunoglobulin was calculated as 1.4 X 10(11) L/mol, which suggests that the immunoglobulin was an autoantibody. The assay may detect 1 microgram of enzyme per liter in a 100- microL sample. CK-MM and CK-MB isoenzymes did not cross react in the assay. Within-assay precision (CV) was 5%; between-assay precision was 14 and 15% at 6 and 14 microgram of CK-BB per liter, respectively. Of 45 sera from blood donors, 95% contained < 1.8 microgram of CK-BB per liter, whereas of 19 sera from unselected patients the corresponding figure was 2.8 microgram/L.