Outcome variables in ankylosing spondylitis: Evaluation of their relevanceand discriminant capacity

The clinical status of ankylosing spondylitis (AS) can be defined by severa l domains (e.g., pain, function, metrology, laboratory) and subcomponents w ithin each domain (e.g., pain using visual analog scale, Schober's within m etrology). Our aim was (1) To define groups of highly correlated variables in order to determine the most relevant; and (2) to evaluate the capacity o f different clinical and biological variables that best discriminate betwee n placebo and active nonsteroidal drugs in AS. Patients with active AS (n = 423) were followed prospectively over 6 weeks while receiving placebo (n = 121) or active nonsteroidal antiinflammatory drugs (n = 352). Eighteen var iables were studied, including global assessment, pain, stiffness, function al indices, metrology, disease activity index, and laboratory markers. Stat istics included (1) Evaluation of the relevance of the different domains by multivariate analysis (CART tree-structure classification; variable cluste ring); and (2) evaluation of the discriminant capacity by univariate analys is [i.e., differences in the standardized response mean (SRM) (mean change/ SD) between placebo and active drug. A value greater than or equal to 0.60 was considered relevant]. Four clusters were identified (patient's subjecti ve perception, inflammatory symptoms, metrology, laboratory data) with mult iple correlation R-2 revealing the most relevant variables to be the Bath A nkylosing Spondylitis Functional Index (BASFI; 0.75), night pain (0.62), Sc hober's test (0.58), and platelet count (0.55), respectively, within each c luster In terms of discriminant power (SRM) the patient perceived global st atus (0.84), lumbar pain (0.73), night pain (0.71), physician global assess ment (0.66), and BASFI (0.65) were most relevant in the univariate analysis . Among the 4 most relevant domains are subjective perception, inflammatory symptoms, metrology, and laboratory. Multivariate analysis of the data rev eals that the spinal pain and the patient global assessment are the variabl es that best discriminate between placebo and active nonsteroidal drug in s hort term studies.