MB-Creatine Phosphokinase Isoenzyme Elevation Not Diagnostic of Myocardial Infarction

To the Editor.— We disagree with the conclusion of Roberts and Sobel (Archives136:421-424, 1976) that increase of the MB isoenzyme of creatine phosphokinase (MB-CPK) in the serum is a specific indicator of myocardial injury. They did demonstrate the absence of MB-CPK from normal skeletal muscle, in agreement with others, 1,2 but they did not consider abnormal skeletal muscle, which is known to produce MB-CPK. Substantial amounts of MB (10% to 30% of total CPK) are present in skeletal muscle biopsy specimens of certain patients with active myopathy, including all of those with Duchenne muscular dystrophy, in some patients with chronic denervation, and occasionally in other patients with other neuromuscular disorders (eg, myotonic atrophy). 1,2 In our own study, as in studies by others, 3 patients with various neuromuscular diseases have shown serum MB-CPK constituting significant fractions of total serum CPK activity: especially Duchenne dystrophy and polymyositis (5% to 30%),