Low Serum Creatine Kinase Activity

Subnormal activity of creatine kinase (CK, ATP: creatine N-phosphotransferase, EC 2.7.3.2) in serum has been observed in a variety of clinical conditions. Subnormal activity may be found as a consequence of diminished efflux of the muscle enzyme into serum from reduced physical activity caused by illness or advanced age or may result from reduced muscle mass accompanying muscle wasting or cachectic states. Low serum CK values reported in acute viral hepatitis have been explained on the basis of reduced physical activity because these patients have been confined to bed for therapeutic reasons or because of the severity of the illness (1). Low CK activities in patients with alcoholic liver disease (2) have been considered to reflect the reduced muscle mass commonly observed in alcoholics. Reduced serum CK activity has also been observed in patients with connective tissue disease unassociated with diminished physical activity (3), but the cause of this is obscure. Diminished enzyme efflux from altered muscle membrane permeability may account for the low serum CK values reported in Cushing’s disease or found in patients on steroid therapy (4) and in thyrotoxicosis. A similar mechanism may be responsible for low serum CK in patients receiving estrogen therapy, estrogen-containing contraceptives, or tamoxifen (5).