Total creatinine kinase and isoenzyme MB activity in serum and skeletal muscle of a patient with dermatomyositis.

Total creatine activity was greatly increased, and creatine kinase MB isoenzyme was measurable in the serum of a patient with dermatomyositis. In contrast, total CK activity in an extract of skeletal muscle from this patient was markedly decreased as compared with normal muscle extracts, but contained about the normal proportion of the MB isoenzymes, 1.5%. We believe skeletal muscle to be the source of the MB isoenzyme in the serum, because the patient's electrocardiographic pattern was normal and she had no signs or symptoms of myocardial infarction.