Disease activity in ankylosing spondylitis: selection of a core set of variables and a first set in the development of a disease activity score.

A large number of variables are available for the assessment of disease activity in ankylosing spondylitis (AS). The aim of this study was to evaluate the validity of commonly used variables, to select a core set of valid variables for disease activity and finally to compute an AS disease activity score (AS-DAS). Data from two longitudinal studies were used. Principal component analysis and reliability analysis resulted in 11 factors: cervical mobility, lumbar flexion, subjective complaints, functional index (FI), enthesis index (EI), inflammatory response, IgA, IgM, root joints, swollen joints and spinal mobility. Based on discriminating power, reproducibility and correlation with disease duration, seven single variables were selected. In a subsequent discriminant analysis, an AS-DAS was computed of five variables, i.e. subjective complaints, FI, EI, root joints and C-reactive protein, which should be validated in the future. A core set of process variables solves the problem of multiple testing in clinical trials, and improves comparability.