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

Phys Ther. 2020 Apr 17;100(4):728-738. doi: 10.1093/ptj/pzaa004.


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.

Trial registration: NCT02356874.

Publication types

  • Multicenter Study
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Exercise Therapy*
  • Female
  • Humans
  • Male
  • Middle Aged
  • Patient Reported Outcome Measures
  • Physical Functional Performance*
  • Sample Size
  • Spondylarthritis / physiopathology*
  • Spondylarthritis / rehabilitation

Associated data