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.
Methods: In a comparative study serum levels of sarcoplasmic proteins indicating muscle cell leakage, creatinine, and C reactive protein (CRP) were measured. Fifty eight AS patients with a mean disease duration of 22 (SD 11) years and 58 age and sex matched controls (without back complaints) were included.
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) micromol/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.