O08 Development and validation of a new tool to assess inflammatory foot disease activity in RA: the Rheumatoid Arthritis Foot Disease Activity Index
Authors: Hoque Anika 
Published: April 2020

Abstract

Background
Rheumatoid arthritis (RA) commonly affects the feet, resulting in pain, walking difficulties, and disability. Omission of foot joints from the disease activity score-28 (DAS-28) may lead to underestimation of foot disease and suboptimal medical and non-medical management. Therefore, the overall objective of this study is to evaluate the measurement properties of the Rheumatoid Arthritis Foot Disease Activity Index-5 (RADAI-F5), a newly developed 5-item (0-10 numerical rating scale) patient-reported outcome measure (PROM) with a summary score (0-10) for measuring foot disease activity in people with RA.

Methods
Participants with RA recruited from NHS rheumatology outpatient clinics completed the RADAI-F5, a self-modified Rheumatoid Arthritis Disease Activity Index (mRADAI-5) which is a self-reported measure of global disease activity, the Foot function index (FFI), Foot Impact Scale impairment/footwear (FIS-IF) and activity/participation (FIS-AP) subscales. DAS-28 was also recorded for each participant whenever possible. Participants completed RADAI-F5 again at 1 week to allow evaluation of 1-week reproducibility. Construct validity was evaluated using Spearman’s rho to test a priori hypotheses for expected strength of associations between the RADAI-F5 and the alternative disease activity and foot-related disability scales at baseline. Internal consistency was evaluated using Cronbach’s alpha. One-week reproducibility was evaluated using the intraclass correlation coefficient (ICC), 90% smallest detectable change (SDC90), and 95% limits of agreement (LOA). Content validity was evaluated using 5-point Likert scales for readability and relevance.

Results
Of 142 respondents, 103 were female and 36 were males with a mean (SD) age of 55 years (12.5) and mean (SD) RA disease duration of 39 months (70.4). Mean (SD, range) RADAI-F5 scores for the sample were 5.02 (2.47, 9.2). Associations were largely consistent with a priori hypotheses for construct validity. Strong associations were observed between the RADAI-F5 and MRADAI-5 (0.789, CI 0.73-0.85), the FFI (0.713, CI 0.62-0.79) and FIS-IS (0.695, CI 0.66-0.82) (p &lt; 0.001). Moderate associations were observed with the FFI-AP (0.478, p &lt; 0.001, CI 0.37-0.63). A weak association was observed between the RADAI-F5 and the DAS-28 (0.379, p &lt; 0.001, CI 0.26-0.57). The RADAI-F5 demonstrated high internal consistency (Cronbach’s Alpha=0.82), and floor and ceiling effects were both absent. The RADAI-F5 demonstrated good reproducibility (ICC=0.868, p &lt; 0.001, CI 0.80-0.91) and the value for SDC90 was 2.26. The upper and lower bounds for 95% LOA were -2.57 to 2.80, with 97% of scores observed within these bounds. Content validity was confirmed with 82% and 84% of participants rating the instrument as relevant and easy to understand respectively. The median time for completion was 5 minutes.

Conclusion
The RADAI-F5 is a highly valid and reliable PROM for measuring foot disease activity in RA patients. Furthermore, the RADAI-F5 appears to be feasible for use in clinical practice and can be used as an adjunct to the DAS28 to measure foot disease activity.

Disclosures
A. Hoque: Grants/research support; Stipend from Versus Arthritis to complete the Versus Arthritis MSK internship 2019 at Glasgow Caledonian University.