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

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.

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.

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.

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.

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.

This study was limited by its moderate sample size.

Our findings suggest that ASPI is preferable over BASFI when evaluating physical function after exercise interventions in patients with axSpA.