To assess the validity of the rheumatoid arthritis impact of disease (RAID) for measuring disease activity of rheumatoid arthritis (RA) and to determine cut-off values for defining the disease activity states.
A total of 622 RA patients from an European database have been included. Cross-validation was based on assessment of convergent and discriminant validity. Optimal cut-offs were determined against external criteria by calculating the respective 25th and 75th percentiles mean values of RAID. External criteria included definitions for remission (REM), low disease activity (LDA), moderate disease activity (MDA) and high disease activity (HDA), cut-offs of the 28-joint disease activity score-C-reactive protein (DAS28-CRP) score.
The RAID showed a moderate degree of correlation with respect to DAS28-CRP (rho=0.417; P<0.0001). The receiver operating characteristic (ROC) curves to discriminate the ability of RAID to distinguish patients with active and non-active disease was very good with an area under the curve (AUC) of 0.847 (95% confidence interval [CI]: 0.816 to 0.878; P<0.0001). Based on the distributions of RAID in the different disease activity groups, we propose the following cut-off values for REM: RAID ≤3; for LDA: RAID >3 and ≤4; for MDA: RAID >4 and ≤6; for HDA: RAID >6. Mean RAID differed significantly between patients classified as REM, LDA, MDA or HDA (P=0.001).
The cut-offs revealed good measurement characteristics in cross-validation analysis, had great discriminatory performance in distinguishing patients with different levels of disease activity and are suited for widespread use in everyday practice application and research.