Measuring Inflammatory Foot Disease in Rheumatoid Arthritis
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.

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.

--Of 142 respondents, 103 were female, with a mean ± SD age of 55 ± 12.5 years and median RA disease duration of 10 months.

--Theoretically consistent associations confirming construct validity were observed with mRADAI-5 (0.789), FFI (0.713), FIS impairment/footwear (0.695), FIS activity/participation (0.478), and the DAS28-ESR (0.379).

--The RADAI-F5 demonstrated high internal consistency and good reproducibility, smallest detectable change 2.69. Content validity was confirmed, with 82% rating the instrument relevant and easy to understand.

To summarize, the RADAI-F5 is a valid, reliable, responsive, and clinically practical patient-reported outcome measure for assessing RA foot disease activity.