Macro-creatine kinase co-migrating with CK-MB on electrophoresis.

Certain types ofmacro-CK, type 1, have been reported to cause false increases in CK-MB by co-migration on electrophoresis (1-3). I report two such cases in which the macroCK was an IgA with anti-CK-BB specificity. According to methods proposed by Medeiros et al. (1), immunoprecipitation and subsequent analysis by elecrophoresis was used to separate interfering macre-CK from CK-MB. CK electrophoresis (Corning, Palo Alto, CA), after incubation of serum with Beckman ICS anti-IgG, anti-IgA, and anti-IgM (Beckman, Inc., Brea, CA), revealed a marked diminution ofapparent CK-MB activity only with anti-IgA. Incubation of serum with anti-CK-BB and anti-CK-MM (Cambridge Medical Diagnostics, Billerica, MA) and subsequent electrophoresis demonstrated a marked decrease in the apparent CK-MB fraction only with anti-CK-BB. No detectable CK-MB was found with the Tandem-E CK-MB assay (Hybritech, Inc., San Diego, CA). I corroborate the results of others and conclude that certain type 1 macro-CK species may cause a falsely increased CK-MB as determined by electrophoresis. However, it has been previously suggested that a proportion of apparent CK-MB activity >40% should alert one to the presence of this interference (1). Samples from these cases contained lower percentages, demonstrating the uncertainty of this criterion in predicting this interference. Expressed in absolute units, apparent CK-MB remained stable over an extended period of time, demonstrating the interference more clearly (Table 1). One further aid in suspecting macro-CK interference with CK-MB on electrophoresis may be the presence of a second macro-CK appearing between CK-MM and CK-MB or as a shoulder on the CK-MM peak. Both of these cases had patterns of this nature, as did at least three other reported cases (2, 3). At present, the only clinically available assays specific enough for quantifying CK-MB in these patients are the Tandem-E CK-MB (Hybritech) or the QuiCK-MB (International Immunoassay Laboratories). Immunoinhibition