OBJECTIVES There is a lack of unanimity about (increased) serum levels of creatine kinase (CK) in patients with ankylosing spondylitis (AS), perhaps because of the inclusion of inappropriate controls. Therefore, serum levels of biochemical markers of muscle origin were assessed in AS patients compared with controls.

RESULTS Lower serum levels in AS patients compared with controls were found for CK (mean (SD): 46 (21) v 76 (44) IU/l; p<0.001), aldolase (0.43 (0.36) v 0.58 (0.32) IU/l; p=0.001), creatinine (91 (13) v 96 (11) μmol/l; p=0.02), alanine aminotransferase (2.8 (1.5)v 4.1 (2.9) IU/l; p=0.001) and aspartate aminotransferase (7.0 (2.7) v 8.4 (3.5) IU/l; p=0.02). Also the lean body mass, as estimated by a formula using height, weight, age and sex, showed lower values in patients versus controls (56 (9) v 59 (9) kg; p=0.004), but creatinine clearance (by Cockcroft and Gault formula) was not different (p=0.48). Partial correlation coefficients adjusted for age and sex showed that CRP levels correlated negatively with CK and aldolase levels in AS patients (r= −0.48, p<0.001 and r= −0.37, p=0.005, respectively).

CONCLUSION Serum levels of biochemical markers of muscle origin were lower in AS patients compared with controls. Patients with active AS, as reflected by high CRP levels, may have an increased protein degradation, predominantly in skeletal muscle.