The Flare-OA-16 questionnaire measuring flare in knee and hip osteoarthritis in the patient perspective: scale reduction and validation using a Rasch model

Abstract Background The recent Flare-OA questionnaire measuring flare in knee and hip osteoarthritis (OA) (19 items in 5 dimensions, numerical rating scale) showed good psychometric properties along with classical test theory. This study aimed to determine its scaling properties by Rasch analysis and to present evidence for a refined scalable version. Methods The participants were 398 subjects with mean age 64 years (SD = 8.1), 70.4% women, recruited from Australia, France, and the United States, with clinically and radiologically symptomatic knee or hip OA, who completed an online survey. The sample was split into derivation and validation subsamples, stratified by country and joint. Rasch analysis examined differential item functioning (DIF) for sex, age, country and joint. A confirmatory factor analysis (CFA) and a convergent validity were performed to document the psychometric properties of the short version. Results To fit the Rasch model, we reordered thresholds of answering modalities when necessary. Two items were removed. A local dependency between 2 items was solved by combining items modalities into a super-item. A uniform DIF (expected and non-removed) was identified for one item that was split by joint, and a non-uniform DIF for one item for age and country (removed in this case). The person-item threshold distribution showed a well focused scale; the CFA and convergent validity showed good fit indicators for the short version. Conclusions The Rasch analysis was helpful in guiding the decision to refine the measurement instrument. After analysis, the Flare-OA-16 questionnaire self-report questionnaire is available for use in clinical research.