Omission of foot joints from composite global disease activity indices may lead to underestimation of foot and overall disease in rheumatoid arthritis (RA) and under-treatment. The aim of this study was to evaluate the measurement properties of the Rheumatoid Arthritis Foot Disease Activity Index-5 (RADAI-F5), a newly developed patient-reported outcome measure for capturing foot disease activity in people with RA.
Participants with RA self-completed the RADAI-F5, modified Rheumatoid Arthritis Disease Activity Index (mRADAI-5), Foot Function Index (FFI), and Foot Impact Scale (FIS) impairment/footwear and activity/participation subscales. The 28-joint Disease Activity Score using the erythrocyte sedimentation rate (DAS28-ESR) was also recorded. Subgroups completed the RADAI-F5 at 1 week and 6 months. Psychometric properties, including construct, content and longitudinal validity, internal consistency, 1-week reproducibility, and responsiveness over 6 months were evaluated.
Of 142 respondents, 103 were female, with a mean ± SD age of 55 ± 12.5 years and median RA disease duration of 10 (interquartile range 3.6-20.8) months. Theoretically consistent associations confirming construct validity were observed with mRADAI-5 (0.789 [95% confidence interval (95% CI) 0.73, 0.85]), FFI (0.713 [95% CI 0.62, 0.79]), FIS impairment/footwear (0.695 [95% CI 0.66, 0.82], P < 0.001), FIS activity/participation (0.478 [95% CI 0.37, 0.63], P < 0.001), and the DAS28-ESR (0.379 [95% CI 0.26, 0.57], P < 0.001). The RADAI-F5 demonstrated high internal consistency (Cronbach's α = 0.90) and good reproducibility (intraclass correlation coefficient = 0.868 [95% CI 0.80, 0.91], P < 0.001, smallest detectable change = 2.69). Content validity was confirmed, with 82% rating the instrument relevant and easy to understand.
The RADAI-F5 is a valid, reliable, responsive, clinically feasible patient-reported outcome measure for measuring foot disease activity in RA.