Measuring inflammatory foot disease in rheumatoid arthritis: development and validation of the Rheumatoid Arthritis Foot Disease Activity Index‐5

Abstract

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 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. 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 impairment/footwear and activity/participation subscales (FIS‐IF and FIS‐AP). The 28 joint Disease Activity Score (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 (standard deviation [SD]) age of 55 years (12.5) and median (interquartile range [IQR]) RA disease duration of 10 (3.6‐20.8) months. Theoretically consistent associations confirming construct validity were observed with mRADAI‐5 (0.789, 95% confidence interval [CI] 0.73‐0.85), FFI (0.713, CI 0.62‐0.79) and FIS‐IF (0.695, CI 0.66‐0.82) (p<0.001); FFI‐AP (0.478, p<0.001, CI 0.37‐0.63) and the DAS28‐ESR (0.379, p<0.001, CI 0.26‐0.57). The RADAI‐F5 demonstrated high internal consistency (Cronbach’s Alpha=0.90) and good reproducibility (ICC=0.868, p<0.001, CI 0.80‐0.91; smallest detectable change=2.69). Content validity was confirmed with 82% rating the instrument relevant and easy to understand. Conclusion The Rheumatoid Arthritis Foot Disease Activity Index‐5 is a valid, reliable, responsive, clinically feasible PROM for measuring foot disease activity in RA.