Objectives: To develop a new index for disease activity in ankylosing spondylitis (ASDAS), that is truthful, discriminative, and feasible, and includes domains/items that are considered relevant by patients and physicians.
Methods: Eleven candidate variables covering 6 domains of disease activity, selected by ASAS-experts in a Delphi exercise, were tested in a 3-step approach, similar to the methodology used for the disease activity score in rheumatoid arthritis. Data on 708 patients included in ISSAS (International Study on Starting TNF-blocking agents in Ankylosing Spondylitis) were used. Cross-validation was done in the OASIS cohort (Outcome in Ankylosing Spondylitis International Study).
Results: Principal component analysis revealed 3 factors with Eigen values > 0.75: “patient-assessments”, “peripheral joint assessments” and “acute phase reactants”. Discriminant function analysis resulted in a correct classification in ~72% of the cases (prior probability: ~50%). Regression analysis resulted in an index with 5 variables (total back pain, patient global, duration of morning stiffness, CRP and ESR). Three additional candidate indices were designed by using similar methodology while omitting either ESR or CRP or patient global. All 4 scores correlated with BASDAI (rho: 0.67-0.80), patient- (0.58-0.76) and physician’s global assessment (0.41-0.48) of disease activity. All 4 candidate ASDAS performed better than BASDAI or single-item variables in discriminating between high- and low disease activity state, according to physicians as well as patients in the OASIS cohort.
Conclusion: The first steps in the development of a new assessment tool of disease activity in AS derived four candidate-indices with good face- and construct- validity, and high discriminant capacity.