Defining disease activity in ankylosing spondylitis: is a combination of variables (Bath Ankylosing Spondylitis Disease Activity Index) an appropriate instrument?

Objective. Disease activity has been defined using a self-administered inst rument, focusing on fatigue, axial pain, peripheral pain, enthesopathy and morning stiffness [Bath Ankylosing Spondylitis Disease Activity Index (BASD AI)]. This validated instrument is simple and takes 40 s to complete, but w hether the index is an accurate expression of the component parts, or wheth er additional weighting would enhance its efficacy, is unclear. Methods. Four hundred and seventy-three patients with ankylosing spondyliti s received placebo or active non-steroidal anti-inflammatory drug (NSAID) f or 6 weeks, and changes between entry and completion were captured by BASDA I and the individual components. Principle component analysis (PCA) was use d to explore the best combinations of variables in decreasing order of expl ained total dispersion and to assess whether a single sum (or algebraic exp ression) best defined disease activity status. Results. At entry, the correlation between BASDAI and the first axis was 0. 99, 0.11 with the second, and zero thereafter. Data at study end and relati ng to change revealed a 100% correlation (R = 1) between the first axis and the sum, with zero for the remainder. Conclusions. The data support BASDAI as being a valid and appropriate compo site to define disease activity in ankylosing spondylitis. Developed as a s imple sum of its components, BASDAI has excellent content validity.