Responsiveness and Interpretability of 2 Measures of Physical Function in Patients With Spondyloarthritis.

BACKGROUND: Maintenance or improvement of physical function is an important treatment target in the management of patients with axial spondyloarthritis (axSpA); measurement tools that can detect changes in physical function are therefore important. OBJECTIVES: The objective of this study was to compare responsiveness and interpretability of the patient-reported Bath Ankylosing Spondylitis Functional Index (BASFI) and the Ankylosing Spondylitis Performed-Based Improvement (ASPI) in measuring change in physical function after exercise in patients with axSpA. DESIGN: This was a sub-study of 58 patients nested within a randomized controlled trial comparing the effect of 12 weeks of exercise with usual care. METHODS: Responsiveness and interpretability were assessed according to the Consensus-based Standards for the selection of health status Measurement Instrument. Responsiveness was assessed by testing 8 predefined hypotheses for ASPI and BASFI. Interpretability was assessed by: (1) using patients' reported change as an anchor ("a little better" = minimal important change) and (2) by categorizing patients with a 20% improvement as responders. RESULTS: For ASPI and BASFI, 5 of 8 (63%) versus 2 of 8 (25%) of the predefined hypotheses for responsiveness were confirmed. The minimal important change values for improvement in physical function were 3.7 seconds in ASPI and 0.8 points (on a scale from 0 to 10) for BASFI. In the intervention group, 21 of 30 (70%) and 13 of 30 (43%) of the patients were categorized as responders measured with ASPI and BASFI, respectively. There was a tendency towards a floor effect in BASFI, as 8 of 58 (14%) patients scored the lowest value at baseline. LIMITATIONS: This study was limited by its moderate sample size. CONCLUSIONS: Our findings suggest that ASPI is preferable over BASFI when evaluating physical function after exercise interventions in patients with axSpA.