AbstractObjectiveThe RA foot disease activity index (RADAI-F5) is a valid, reliable and clinically feasible patient-reported outcome measure (PROM) for the measurement of RA foot disease activity. Further validation of the RADAI-F5 against musculoskeletal ultrasonography (MSUS) for foot disease activity is necessary before clinical implementation. The aim of this study was to examine the construct validity of the RADAI-F5 in relationship to MSUS and clinical examination.MethodsParticipants with RA completed the RADAI-F5. MSUS was used to evaluate disease activity (synovial hypertrophy/synovitis/tenosynovitis/bursitis) and joint damage (erosion) using greyscale (GS) and power Doppler (PD) at 16 regions in each foot, including joints and soft tissues. These same regions were examined clinically for swelling and tenderness. The construct validity of the RADAI-F5 was assessed using correlation coefficients and a priori-specified hypotheses for the strength of associations.ResultsOf 60 participants, 48 were female, with a mean (s.d.) age of 62.6 (9.96) years and median disease duration of 15.49 (interquartile range 6–20.5) years. Theoretically consistent associations confirming construct validity [95% CI] were observed between the RADAI-F5 and MSUS GS (0.76 [0.57, 0.82]; strong), MSUS PD (0.55 [0.35, 0.71]; moderate), MSUS-detected erosions (0.41 [0.18, 0.61]; moderate), clinical tenderness (0.52 [0.31, 0.68]; moderate) and clinical swelling (0.36 [0.13, 0.55]; weak).ConclusionModerate to strong correlations between RADAI-F5 and MSUS demonstrate the good measurement properties of this instrument. With greater confidence in the utility of the RADAI-F5, clinical use of this new instrument as an adjunct to the disease activity score for 28 joints (DAS-28) could help to identify RA patients at risk for poor functional and radiological outcomes.