An immunochemical procedure for determination of creatine kinase 3(1) (serum-specific) isoform in human serum evaluated.

Changes in the proportions of individual isoforms of creatine kinase (CK) in serum promptly reflect both myocardial infarction and coronary reperfusion. A new commercial kit has been introduced for measuring CK-3(1) isoform in serum (ISOFOR-MM, International Immunoassay Labs.). This is an immunochemical assay containing CK-3(1) specific monoclonal antibody, bound to magnetizable particles, used to immunoextract this isoform. The CK activity of the sample is measured before and after immunoextraction and the difference in the two values gives the measure of CK-3(1). Extraction of CK-3(1) was complete at less than or equal to 1200 U/L. Analysis of between-day imprecision gave CV between 2.9-7.9%. The method was not susceptible to interference by CK-3(2) and CK-3(3) isoforms, CK-2 isoenzyme, or mitochondrial CK. Reference interval for CK-3(1) (expressed as percent of total CK-3) was 42-69%. Correlation between percent CK-3(1) by isoform electrophoresis (x) and evaluated procedure (y) was y = 0.83x + 7.6, with r = 0.957 (n = 40). The ISOFOR-MM performed well enough in this evaluation to replace electrophoresis or isoelectric focusing for measurement of CK-3(1) isoform.