Objective: To further document the measurement properties of each domain of the OA of knee and hip quality of life (OAKHQOL) questionnaire by a Rasch analysis.
Methods: The OAKHQOL self-administered questionnaire has been developed to assess health-related quality of life in lower limb OA. Patients with various degrees of severity of knee or hip OA answered the questionnaire. For each domain, their responses to the items were analysed with a Rasch family model, the partial credit model. We examined the fit of data to model expectations, as well as assumptions of unidimensionality and local independence. Invariance was assessed by analysis of differential item functioning (DIF) by sex, age and joint. Analyses used the RUMM2020 software (Rumm Laboratory, Perth, Western Australia).
Results: Responses for 544 patients were analysed: 297 had medically managed OA and 247 were waiting for arthroplasty surgery. For the 40 items of the OAKHQOL, data analysis showed 5 with disordered thresholds and 9 with DIF (5 for joint, 3 for sex and 1 for age). Ten pairs of items showed local dependence and four domains showed unidimensionality. Full-item domains and domains without the misfitted items did not differ in patient-estimates data; therefore any bias at the item level is negligible when considering the domain scores.
Conclusion: The five domains of the OAKHQOL questionnaire show good measurement properties by Rasch analysis and provide valid scales.