An immunoinhibition assay for determination of creatine kinase isoforms in serum

We report a preliminary evaluation of an immunoinhibition assay for creatine kinase isoform quantification. The procedure employs the monoclonal antibody CKM-G01, which inhibits the native M subunit of creatine kinase. The antibody does not inhibit the M subunit modified by removal of lysine by plasma carboxypeptidase N. Residual activity after treatment with the antibody is therefore due to serum delysinated isoforms. The ratio inhibited/residual activity correlated directly with the ratio tissue/serum isoforms. Analysis of the total imprecision of isoform ratio measurement gave a coefficient of variation between 5.9 and 21.1%. Reference intervals for the ratio were 0.14-0.79 in females and 0.19-0.95 in men (p = 0.0046). Analytical and clinical comparison with alternative isoform procedures gave good results, showing that this assay can be used as alternative to the widely accepted electrophoretic method for measurement of the creatine kinase isoform ratio.