Improved radioimmunoassay for creatine kinase isoenzymes in plasma

We describe convenient and relatively rapid procedures for purifying creatine kinase isoenzymes MM, BB, and MB, and their use in an improved radioimmunoassay for creatine kinase isoenzymes in plasma. The modifications include use of: (a) BB with a specific activity of 400 kU/g, which can be labeled with a specific radioactivity of 20 Ci/g; (b) albumin-free purified MB as inhibitor; (c) antiserum to MB creatine kinase; and (d) a second-antibody technique that necessitates only a 15-min incubation. The radioimmunoassay for MB has a sensitivity of 0.2 microgram/L (80 mU/L) and a CV of less than 5%. Plasma MB average 22 (SD 12) microgram/L in 200 normal subjects; 24 (SD 12) microgram/L in 200 patients with chest pain without infarction; and 23 (SD 7) microgram/L in 43 patients with renal disease, whether measured before or after dialysis. Peak values for plasma MB averaged 191 (SD 86) microgram/L in 325 patients with documented myocardial infarction; BB was negligible. Extensive clinical experience indicates the radioimmunoassay to be suitably rapid, highly sensitive, and reliable as a diagnostic assay for MB in plasma.