A comparative study of the usefulness of the Bath Ankylosing Spondylitis Functional Index and the Dougados Functional Index in the assessment of ankylosing spondylitis

To determine whether the Bath Ankylosing Spondylitis Functional Index (BASFI, score 0-10) or Dougados Functional Index (DFI, score 0-40) is superior in measuring physical function in ankylosing spondylitis (AS) we studied 191 consecutive outpatients with AS in the Netherlands, France, and Belgium. The participating centers are secondary and tertiary referral centers. The external criterion for disease activity (DA) was: both patient and physician assessment of disease activity on a visual analog scale (VAS) and the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). The external criterion for damage was 2 radiological scores of the spine; BASRI-s (Bath Ankylosing Spondylitis Radiology Index-spine) and a modified SASSS (Stoke Ankylosing Spondylitis Spine Score). Median scores for BASFI and DFI were 2.5 (range 0-10) and 8.5 (range 0-35), respectively. Spearman correlation coefficient between both indexes was 0.89. The average correlation with disease activity variables was 0.42 for BASFI and 0.41 for DFI. For both BASFI and DFI the correlation with BASRI-s was 0.42 and with SASSS 0.36. When distinguishing between patients with high and low disease activity, sensitivity for both indexes was between 76 and 94%, while specificity was between 66 and 87% for all 3 DA measures, Average misclassification between BASFI, DFI and DA was 23 and 27%, respectively. Both BASFI and DFl correlate equally well with disease activity and damage.