Macrocreatine kinase in plasma: a cause for a false positive CK-MB immunoinhibition test.

The case of a patient with bronchial neoplasm is described. A non-typical form of creatine kinase found in the plasma gave rise to a false positive CK MB test by the immunoinhibition procedure. Electrophoresis on agarose gels beside a normal MM band showed an additional band between the MB and the MM band of a standard mixture. It could be shown by an immunoprecipitation test using precipitating antibodies against CK-MM and CK-BB that the creatine kinase of the abnormal band consists entirely of the MM-type, which after treatment with polyethylene glycol not only became susceptible to inhibiting anti-M antibodies, but also showed the electrophoretic motility of CK-MM. Gel filtration chromatography clearly demonstrated the macromolecular nature of this abnormal creatine kinase.