Raised serum creatine phosphokinase activity in ankylosing spondylitis.

Serum enzyme studies were made on 43 (37 male, 6 female) consecutive patients with ankylosing spondylitis. Serum creatine phosphokinase (CPK) activity was raised above 55 IU/l in 24 (65%) of 37 male patients (range 29-165 IU/l, mean 68) as compared with 2 (4%) out of 47 male controls (range 14-85 IU/l, mean 33: P less than 0-001); levels were greater than 35 IU/l in six (100%) out of six female patients (range 39-100 IU/l, mean 56) as against one (3%) of 35 female controls (range 3-106 IU/l, mean 16; P less than 0-001). The recognized pitfalls in interpreting CPK activity were avoided. In all of sixteen randomly selected patients isoenzyme studies confirmed that muscle is the source of the enzyme. There was a significant correlation between CPK activity and both spinal flexion and the reciprocal of finger-to-floor distance (P less than 0-5 in each case).