Objective: To investigate the responsiveness and discriminative capacity, and the relationship between both, of instruments selected for the disease-controlling antirheumatic therapy (DC-ART) core set by the Assessments in Ankylosing Spondylitis Working Group (ASAS).
Methods: Responsiveness and discriminative capacity of different measures reflecting disease activity and function, either included in the ASAS DC-ART core set or not, were evaluated in a randomized controlled clinical trial comparing etanercept with placebo in patients with ankylosing spondylitis. Guyatt's method was used as the primary analysis for responsiveness, and Student's t-test for discriminative capacity.
Results: At day 28 of therapy, almost all measures indicated moderate to large responsiveness in the etanercept group (Guyatt 0.60-3.11). Some scales of the Short Form 36 (general health, mental component summary, and role emotional), the modified Schober's test, and the Fatigue Severity Scale were not responsive. The results were similar if analyzed at day 112 of therapy. Peripheral joint counts, joint scores, and occiput-to-wall distance could not be evaluated due to a floor effect. In general, the relation between responsiveness and discriminative capacity was strong: Measures that demonstrated high responsiveness also showed high between-group t values.
Conclusion: Measures included in the ASAS DC-ART core set, except modified Schober's test, have good responsiveness and good discriminatory capacity. Some measures could not be evaluated due to a floor effect.