Clinical significance of low creatine phosphokinase values in patients with connective tissue diseases.

We have noted that values for serum creatine phosphokinase (CPK) are frequently low in patients with connective tissue diseases. Geometric mean values were 96.0 units/L among 121 disease-free males and 50.2 units/L among 63 disease-free females. The geometric mean values were 31.7 units/L in 11 males and 35.2 units/L in 49 females with systemic lupus erythematosus; 15.1 units/L in three males and 13.0 units/L in ten females with rheumatoid arthritis; and 41.6 units/L in 19 females with Sjögren's syndrome. Values within the normal range may occur in the face of active myositis in these illnesses. Findings of a low serum CPK values may occur in the absence of steroid treatment. Thus, a low serum CPK value is frequently found in patients with connective tissue diseases and a normal value should not dissuade one from the diagnosis of myositis.