Osteoarthritis (OA) is a common chronic condition that affects millions of people around the world [1]. The knee and hip are two of the joints most frequently affected, and people living with OA can experience intermittent exacerbations of their symptoms or ‘flares’ [2,3]. Following OMERACT Handbook methodology, the following definition for the target construct of flare in OA knee and/or hip, “a transient state, different from the usual state of the condition, with a duration of a few days, characterized by worsening of pain, swelling, stiffness, impact on sleep, activity, functioning, and psychological aspects that can resolve spontaneously or lead to a need to adjust therapy” was endorsed by the OMERACT 2018 meeting [4]. The OMERACT Flares in Osteoarthritis Working Group (WG) then conducted a literature review strategy, qualitative interviews with patients and health professionals, a series of surveys and international voting at the inaugural virtual OMERACT meeting in 2020 to endorse five core domains: pain, swelling, stiffness, psychological aspects, and impact of symptoms [5].

The measurement of these five domains is needed to capture the occurrence of the targeted construct of a flare in OA. To find instruments that truly reflect the construct, i.e. measuring all core domains, the OMERACT group has developed methodological steps to accomplish this task. In this study, we present the results obtained following these steps to select candidate instrument(s) to assess flares in osteoarthritis.