SUMMARYAn increased serum level of the MB isoenzyme of creatine kinase (CK‐MB) is a useful marker for acute myocardial infarction. Although described extensively in clinical chemistry literature, there is little information in standard medical references about false positives for this test. We report two cases where high levels of measured CK‐MB activity were in fact due to another form of CK, associated with internal malignancy.
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