Comparative responsiveness of 3 functional indices in ankylosing spondylitis

Objective. To compare the responsiveness of the Bath Ankylosing Spondylitis Functional Index (BASFI), the Dougados Functional Index (DFI), and the ank ylosing spondylitis-specific version of the Health Assessment Questionnaire (HAQ-S). Methods. 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 f unctional performance. Instrument responsiveness was examined using 4 stati stics including standardized response mean (SRM), effect size (ES), the Guy att method, and the variance method. Ceiling and floor effects were visuali zed using histograms for cross sectional and time-path diagrams for longitu dinal analysis. Results. The BASFI ranked superior compared to the DFI and HAQ-S in detecti ng both improvement and deterioration of functional performance. All 4 resp onsiveness 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 pa tterns. Conclusion. The BASFI appears to be more responsive than the DFI and the HA Q-S in situations of both improvement and deterioration of functional perfo rmance. However, addition of particular difficult items may improve the per formance of the BASFI as well as of the other 2 instruments.