Objectives: To investigate construct validity and responsiveness of the novel ankylosing spondylitis disease activity score (ASDAS) in patients with spondyloarthritis (SpA).
Methods: In a 46 weeks prospective, longitudinal multi-center study of 60 SpA patients (80% men, median age 40 years (range 21-62)) treated with tumor-necrosis-factor-alpha (TNF-α) inhibitors (infliximab (n=41), etanercept (n=13), adalimumab (n=6)) responsiveness of ASDAS, conventional clinical measures of disease activity and treatment response, and the Berlin magnetic resonance imaging (MRI) sacroiliac joint (SIJ) and lumbar spine inflammation scores were compared.
Results: After 22 weeks, 58.3% of the patients were clinical responders (50% or 20mm reduction in BASDAI). At baseline, clinical responders had significantly higher ASDAS (median 4.15 (range 1.98-6.04), p=0.008) compared with non-responders (2.99, 2.05-6.19). Changes in ASDAS correlated with changes in clinical measures of disease activity (incl. BASDAI (rho=0.76) and CRP (0.79)), MRI SIJ inflammation (0.46) and MRI total inflammation scores (0.34). Patients with higher BASDAI or ASAS responses obtained more profound reductions in ASDAS. ASDAS demonstrated the highest responsiveness with an effect size of 2.04 and a standardized response mean of 1.45, whereas BASDAI (1.86; 1.36) and CRP (0.63; 0.70) were less responsive. Linear regression demonstrated that a change in BASDAI of 20mm or 50% corresponded to change in ASDAS of 1.38 and 1.95, respectively.
Conclusion: ASDAS demonstrated construct validity and high responsiveness during treatment with TNF-α inhibitors in SpA patients. The proposed thresholds for disease activity and treatment response need further validation.