DISEASE-ACTIVITY IN ANKYLOSING-SPONDYLITIS - SELECTION OF A CORE SET OF VARIABLES AND A FIRST STEP IN THE DEVELOPMENT OF A DISEASE-ACTIVITYSCORE

A large number of variables are available for the assessment of diseas e activity in ankylosing spondylitis (AS). The aim of this study was t o evaluate the validity of commonly used variables, to select a core s et of valid variables for disease activity and finally to compute an A S disease activity score (AS-DAS). Data from two longitudinal studies were used. Principal component analysis and reliability analysis resul ted in 11 factors: cervical mobility, lumbar flexion, subjective compl aints, functional index (FI), enthesis index (EI), inflammatory respon se, IgA, IgM, root joints, swollen joints and spinal mobility. Based o n discriminating power,reproducibility and correlation with disease du ration, seven single variables were selected. In a subsequent discrimi nant analysis, an AS-DAS was computed of five variables, i.e. subjecti ve complaints, FI, EI, root joints and C-reactive protein, which shoul d be validated in the future. A core set of process variables solves t he problem of multiple testing in clinical trials, and improves compar ability.