Identifying the primary outcome for a randomised controlled trial in rheumatoid arthritis: the role of a discrete choice experiment.

Background: This study sought to establish the preferences of people with Rheumatoid Arthritis (RA) about the best outcome measure for a health and fitness intervention randomised controlled trial (RCT). The results of this study were used to inform the choice of the trial primary and secondary outcome measure.

Methods: A discrete choice experiment (DCE) was used to assess people's preferences regarding a number of outcomes (foot and ankle pain, fatigue, mobility, ability to perform daily activities, choice of footwear) as well as different schedules and frequency of delivery for the health and fitness intervention. The outcomes were chosen based on literature review, clinician recommendation and patients' focus groups. The DCE was constructed in SAS software using the D-efficiency criteria. It compared hypothetical scenarios with varying levels of outcomes severity and intervention schedule. Preference weights were estimated using appropriate econometric models. The partial log-likelihood method was used to assess the attribute importance.

Results: One hundred people with RA completed 18 choice sets. Overall, people selected foot and ankle pain as the most important outcome, with mobility being nearly as important. There was no evidence of differential preference between intervention schedules or frequency of delivery.

Conclusions: Foot and ankle pain can be considered the patient choice for primary outcome of an RCT relating to a health and fitness intervention. This study demonstrated that, by using the DCE method, it is possible to incorporate patients' preferences at the design stage of a RCT. This approach ensures patient involvement at early stages of health care design.