Effect of creatine kinase-MM subtype composition on a CK-MB immunoinhibition assay

Immunoinhibition (INH) by use of polyclonal anti-human CK-M antibody may be used to measure CK-MB in serum. Previous studies have shown that inhibiting antibodies prepared against purified muscle extracts may inhibit CK-MM by greater than 99%. Using patients' sera and muscle homogenates incubated with human serum, we studied the effect of CK-MM subtype composition on an INH assay. We found that with increasing time from the CK-releasing event, e.g., myocardial infarction, or with longer in vitro incubation, the proportion of CK-MM1 increased and the proportion of uninhibited CK-MM increased from 0.2% to 0.7-0.8%. As a consequence, CK-MB activity may be overestimated by as much as 1.6% of total CK when uncorrected INH results are used. Inhibition was maximal in samples containing 100% CK-MM3, the tissue subtype. Because of the time-dependent change in CK-MM subtypes, published results for INH from studies in which CK-MM purified from muscle was used may not be directly applicable to clinical specimens.