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 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.