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We concur fully with the comments on interpretation of CK-MB isoenzymes in Dr Dubin's communication: (1) The CK-MB isoenzyme fraction may be positive in the qualitative assay system from extensive skeletal injury alone. (2) Appropriate dilution of total CK activity before isoenzyme separation will render the CK-MB band negative in the majority of cases because CK-MB is diluted below the level of sensitivity in the assay system. (3) Persistence of the CK-MB band after dilution is suggestive of concurrent myocardial injury, as it represents a greater percentage of CK activity in the MB fraction. We agree that a negative CK-MB band after dilution does not exclude myocardial injury, as extensive skeletal damage may mask the effect of lesser myocardial involvement. This point deserves the emphasis that he brings to it.
Siegel AJ, Dawson DM. MB Fraction of Creatine Kinase-Reply. JAMA. 1981;245(23):2393. doi:10.1001/jama.1981.03310480013008