ObjectiveAdvances in the field of axial spondyloarthritis (axSpA) and the methodology to develop core sets have led the Assessment of SpondyloArthritis international Society (ASAS) group to update the ASAS–Outcomes in Rheumatology (OMERACT) core set. An important aspect was to ensure it would be applicable to the entire spectrum of axSpA. The first step was to define the most relevant disease domains.MethodsA 3-round Delphi survey was conducted to gather opinions of 188 patients and 188 axSpA experts to define the most relevant disease domains to be included in the core set. The Delphi survey evaluated 2 separate research settings: (1) studies assessing symptom-modifying therapies; and (2) studies evaluating disease-modifying therapies. Importance of the domains was rated on a 1–9 Likert scale. A domain was considered for inclusion if, for both stakeholder groups, ≥ 70% of participants scored the domain as critical (7–9) and ≤ 15% scored it as not important (1–3) after 3 rounds.ResultsA total of 132 (70%) patients and 135 (72%) experts completed at least 1 round. After 3 rounds, 7 domains (pain, physical function, stiffness, disease activity, mobility, overall functioning and health, peripheral manifestations) were selected for the symptom-modifying therapies setting. For the disease-modifying therapies setting, 6 domains (physical function, disease activity, mobility, structural damage, extramusculoskeletal manifestations, peripheral manifestations) were selected. All domains selected by experts were also selected by patients. Patients selected all offered domains except emotional function.ConclusionThis study provides the domains selected by patients and axSpA experts that should be considered for the core set for axSpA.