To the editor: The association between lupus myositis and myocarditis reported by Borenstein and co-workers in the November issue (1) is very interesting. I wish to comment on the significance of the serum creatine kinase (CK)-cardiac (MB) isoenzyme elevation in their patient and on their suggestion that CK isoenzyme determination may be of value in the diagnosis of lupus myocarditis.
A small amount of CK-MB is found in skeletal muscle by tissue analyses (2). In addition, in healthy control subjects, up to 2% of serum CK may be in the MB form (3). Therefore, the total CK of 1730 U/litre