Abstract
OBJECTIVE: To assess the feasibility, reliability, and validity of the time trade off (TTO) in patients with rheumatoid arthritis (RA). METHODS: The TTO was applied in 194 patients with RA with increasing difficulty in performing activities of daily living. The test-retest reliability was determined in 35 of these patients and was calculated by the intraclass correlation coefficient (ICC). Construct validity was evaluated with the following sets of variables: measures of utility (rating scale), quality of life (RAND 36 item Health Status Survey (RAND-36) and RAQoL), functional status (Health Assessment Questionnaire, grip strength, and walk test), and disease activity (doctor's global assessment, disease activity score, pain, and morning stiffness). RESULTS: Ten patients (5%) did not complete the TTO. The median value of the TTO was 0.77 (range 0.03-1. 0). The test-retest ICC of the TTO was 0.85 (p<0.001). Construct validity testing of the TTO showed poor to moderate correlations (Spearman's r(s) between 0.19 and 0.36, p<0.01) with all outcome measures except for the subscale role limitation (physical problem) of the RAND-36, the walk test, the doctor's global assessment of disease activity, and morning stiffness. Multiple regression analysis showed that only 17% of the variance of the TTO scores could be explained. CONCLUSIONS: The TTO method appeared to be feasible and reliable in patients with RA. The poor to moderate correlations of the TTO with measures of quality of life, functional ability, and disease activity suggest that the TTO considers additional attributes of health status. This may have implications for the application of the TTO in clinical trials in patients with RA.