Searching for optimal rating scales in the Bath Ankylosing Spondylitis Functional Index (BASFI) and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI).

The comparison of the performance of the numerical rating scale (NRS) versus visual analog scale (VAS) in the Bath Ankylosing Spondylitis Functional Index (BASFI) and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) showed that the 11-point NRS is psychometrically superior to the 10-cm VAS. This finding is in agreement with previous studies and the recommendation by the Assessment of SpondyloArthritis international Society. To illustrate the functioning of the response categories of both BASFI and BASDAI, we analyzed the rating scales (using the Rasch rating scale model) in patients with ankylosing spondylitis. Our results have shown that the 11 categories available in the 0-10 NRS version of both BASFI and BASDAI exceed the number of levels of a construct that participants can discriminate. This indicates the need for improving the metric quality of both rating scales by appropriately reducing the number of categories.