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 instrument, focusing on fatigue, axial pain, peripheral pain, enthesopathy and morning stiffness [Bath Ankylosing Spondylitis Disease Activity Index (BASDAI)]. This validated instrument is simple and takes 40 s to complete, but whether the index is an accurate expression of the component parts, or whether additional weighting would enhance its efficacy, is unclear. Methods. Four hundred and seventy-three patients with ankylosing spondylitis received placebo or active non-steroidal anti-inflammatory drug (NSAID) for 6 weeks, and changes between entry and completion were captured by BASDAI and the individual components. Principle component analysis (PCA) was used to explore the best combinations of variables in decreasing order of explained total dispersion and to assess whether a single sum (or algebraic expression) 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 relating 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 composite to define disease activity in ankylosing spondylitis. Developed as a simple sum of its components, BASDAI has excellent content validity.