Objective: The goal of the Rheumatoid Arthritis Disease Activity Index (RADAI) is to provide an easy to use assessment of disease activity. It is a self-administered questionnaire that combines five items into a single index: current and past global disease activity, pain, morning stiffness and a joint count.
Methods: A sample of 484 rheumatoid arthritis (RA) patients was used to assess the internal consistency and the convergent validity of the RADAI. This was achieved by calculating Cronbach's alpha and RADAI item and total score correlations with core set measures and DAS28.
Results: Cronbach's alpha was 0.87, supporting the summation of the items into a single index. The index correlated best with physicians' global assessment (r = 0.59; P < 0.0001), the Health Assessment Questionnaire (r = 0.55; P < 0.0001) and the number of tender joints (r = 0.55; P < 0.0001). Correlation with the erythrocyte sedimentation rate was low (r = 0.27; P < 0.0001). The RADAI and the DAS28 were correlated (r = 0.53; P < 0.0001), but there was low agreement.
Conclusions: The RADAI is valid to assess disease activity in RA patients. However, the RADAI may not automatically replace other measures of disease activity, such as the DAS28.