The clinical status of ankylosing spondylitis (AS) can be defined by several domains (e.g., pain, function, metrology, laboratory) and subcomponents within each domain (e.g., pain using visual analog scale, Schober's within metrology). Our aim was (1) To define groups of highly correlated variables in order to determine the most relevant; and (2) to evaluate the capacity of different clinical and biological variables that best discriminate between 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 variables were studied, including global assessment, pain, stiffness, functional indices, metrology, disease activity index, and laboratory markers. Statistics included (1) Evaluation of the relevance of the different domains by multivariate analysis (CART tree-structure classification; variable clustering); and (2) evaluation of the discriminant capacity by univariate analysis [i.e., differences in the standardized response mean (SRM) (mean change/SD) between placebo and active drug. A value > or =0.60 was considered relevant]. Four clusters were identified (patient's subjective perception, inflammatory symptoms, metrology, laboratory data) with multiple correlation R2 revealing the most relevant variables to be the Bath Ankylosing Spondylitis Functional Index (BASFI; 0.75), night pain (0.62), Schober's test (0.58), and platelet count (0.55), respectively, within each cluster. In terms of discriminant power (SRM) the patient perceived global status (0.84), lumbar pain (0.73), night pain (0.71), physician global assessment (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 reveals that the spinal pain and the patient global assessment are the variables that best discriminate between placebo and active nonsteroidal drug in short term studies.