Comparative responsiveness of 3 functional indices in ankylosing spondylitis

To compare the responsiveness of the Bath Ankylosing Spondylitis Functional Index (BASFI), the Dougados Functional Index (DFI), and the ankylosing spondylitis-specific version of the Health Assessment Questionnaire (HAQ-S).

Using data from a placebo controlled, randomized clinical trial we designed a responsiveness model including both hypothesized improvement (n = 39; continuous diclofenac treatment) and hypothesized deterioration (n = 40; patients who withdrew from the clinical trial because of a flare) of functional performance. Instrument responsiveness was examined using 4 statistics including standardized response mean (SRM), effect size (ES), the Guyatt method, and the variance method. Ceiling and floor effects were visualized using histograms for cross sectional and time-path diagrams for longitudinal analysis.

The BASFI ranked superior compared to the DFI and HAQ-S in detecting both improvement and deterioration of functional performance. All 4 responsiveness statistics consistently confirmed this superiority of the BASFI. However, the median baseline scores of all 3 questionnaires were close to the lower end of the instrument and the score distribution showed skewed patterns.

The BASFI appears to be more responsive than the DFI and the HAQ-S in situations of both improvement and deterioration of functional performance. However, addition of particular difficult items may improve the performance of the BASFI as well as of the other 2 instruments.