Defining domains: developing consensus-based definitions for foundational domains in OMERACT core outcome sets

Objective: To develop a set of detailed definitions for foundational domains commonly used in OMERACT (Outcome Measures in Rheumatology) core domain sets. Methods: We identified candidate domain definitions from prior OMERACT publications and websites and publications of major organizations involved in outcomes research for six domains commonly used in OMERACT Core Domain Sets: pain intensity, pain interference, physical function, fatigue, patient global assessment, and health-related quality of life. We conducted a two-round survey of OMERACT working groups, patient research partners, and then the OMERACT Technical Advisory Group to establish their preferred domain definitions. Results were presented at the OMERACT 2023 Methodology Workshop, where participants discussed their relevant lived experience and identified potential sources of variability giving the needed detail in our domain definitions. Results: One-hundred four people responded to both rounds of the survey, and a preferred definition was established for each of the domains except for patient global assessment for which no agreement was reached. Seventy-five participants at the OMERACT 2023 Methodology Workshop provided lived experience examples, which were used to contextualise domain definition reports for each of the five domains. Conclusion: Using a consensus-based approach, we have created a detailed definition for five of the foundational domains in OMERACT core domain sets; patient global assessment requires further research. These definitions, although not mandatory for working groups to use, may facilitate the initial domain-match assessment step of instrument selection, and reduce the time and resources required by future OMERACT groups when developing core outcome sets.