A Dutch version of the functional index for ankylosing spondylitis: development and validation in a long-term study

A Dutch Functional Index (DFI) for AS has been developed, containing 37 questions in essence this is a modification of a French index with additional questions. Internal consistency, reproducibility, criterion and construct validity and sensitivity to change were studied in different groups of AS patients. In a group of 149 patients the questionnaire was completed and internal consistency calculated (Cronbach's alpha = 0.94) after exclusion of three items. Reproducibility was studied in 39 patients with stable disease; Pearson correlation was 0.94. Criterion validity, against a 'gold standard' i.e. experts' observation of 25 items, was studied in 19 patients; r = 0.85. In a 48-week NSAID study patients were enrolled after a washout period: DFI scores before and after treatment showed significant improvement (P < 0.02). There were 187 DFI with corresponding clinical measurements, correlations varied from -0.30 to 0.68. Estimated measurement-remeasurement correlation was 0.87. The DFI is, thus, a potentially useful instrument, which is valid, reliable and sensitive to the effects of NSAID treatment.