The need for a tool to measure flare in lower limb osteoarthritis (OA of the hip and knee) is justified by the opportunities for development of clinical trials targeting hip and knee OA long-term treatment (slow acting drug, disease modifying drug) and short-term treatment of symptoms. Such treatments are currently under development and will likely bring important changes in the treatment of patients. The development of a Flare-OA tool would be essential to capture the occurrence of flare, and to implement and adapt treatment strategies.

This project was elaborated with the aims to:

1. define the construct through development of a conceptual framework
2. use the conceptual framework to inform the development of a patient-reported outcome measure, Flare-OA tool, for people with OA
3. evaluate the psychometric properties of the new measure using the OMERACT (Outcome Measures in Rheumatology) filter 2.0 and Rasch measurement methods.

Chronologically, the development of a Flare-OA (osteoarthritis) tool will be addressed in the first part of the project (Part 1) and its psychometric validity was evaluated in the second part (Part 2).

Part 1 of the project: The development of the Flare-OA tool will be conducted in several steps, and needs multiple proficiencies in qualitative research (health psychologist), in quantitative research (epidemiologist, psychometrician), in experience of the disease (patients), and in experience of care of the disease (health care professionals).

Two approaches will be taken to define the construct of a flare in lower limb OA: A review of the literature, and the development of a conceptual framework informed by the results of the literature. To help define/conceptualize the construct "Flare in OA" a procedure will be implemented to identify (what are the key variables, or symptoms, that get worse) and clarify (how exactly these variables are described in the literature, by patients, and by clinicians.

Participants :

1. - Patients should be representative of the disease spectrum, i.e. of prevalent cases. They will be recruited in clinical settings and epidemiological sources.
2. - Health care professionals (HCP) will include General Practitioners, Rheumatologists, Rehabilitation specialists, Orthopaedic surgeons, Nurse in general practice or orthopedic wards, Physiotherapists.

Inclusion Criteria:

• with clinical and radiological hip or knee OA confirmed by a physician,
• whatever their status regarding flare of OA. The intention of the Flare-OA tool is not to detect current flare only, but also a flare that may have occurred before the encounter with the health care professional. Therefore patient will not be selected on flare, and patients with flare will not be excluded.

Exclusion Criteria:

• Patients who never experienced nor currently report having a flare of OA
• Patients with both hip and knee OA

Patients with knee or hip osteoarthritis (OA).

France: several sources will be sollicited including:

• investigators in rheumatology and general practitioner network in each participating centre,
• Patients participating in the KHOALA cohort, a representative prevalent case cohort study followed-up in 6 centres.

Australia: patients participating in the online cohort SPARK-web study Canada : patients will be recruited in Toronto osteoarthritis center.