Measuring Inflammatory Foot Disease in Rheumatoid Arthritis: Development and Validation of the Rheumatoid Arthritis Foot Disease Activity Index-5

Objective Omission of foot joints from composite global disease activity indices may lead to underestimation of foot and overall disease in rheumatoid arthritis (RA) and undertreatment. 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 disease activity of the foot in people with RA. Methods 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 count 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. Results Of 142 respondents, 103 were female, with a mean +/- SD age of 55 +/- 12.5 years and median RA disease duration of 10 months (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 alpha = 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. Conclusion The RADAI-F5 is a valid, reliable, responsive, clinically feasible patient-reported outcome measure for measuring foot disease activity in RA.