Measurement properties of the Health Assessment Questionnaire Disability Index for generalized osteoarthritis.

OBJECTIVE Generalized OA (GOA) is highly prevalent in OA. Individuals with GOA typically suffer from limitations of both upper and lower extremity function, yet we lack a validated instrument to assess their activity limitations. An appropriate instrument might be the HAQ Disability Index (HAQ-DI). Therefore the aim of this study was to evaluate the measurement properties of the HAQ-DI in GOA. METHODS Data were used from a randomized controlled trial comparing the effectiveness of two multidisciplinary treatment programmes for patients with GOA. One hundred and thirty-seven of 147 included patients completed a standardized set of questionnaires before and after treatment. Interpretability, validity, reliability and responsiveness of the HAQ-DI were assessed using the Consensus-Based Standards for the Selection of Health Status Measurement Instruments checklist (COSMIN). RESULTS Floor and ceiling effects were present. The content validity was questionable since the HAQ-DI encompasses activities that are either not relevant or too easy to perform as judged by patients and experts. Construct validity was good since 90% of the hypotheses were confirmed. Factor analysis confirmed the unidimensionality of the HAQ-DI (root mean square error of approximation = 0.057, χ(2)/df ratio = 1.48). Cronbach's α was 0.90, confirming internal consistency and the ICC was 0.81, reflecting good reliability. The minimal important change was 0.25 and the smallest detectable change was 0.60. We could not establish the responsiveness of the HAQ-DI. CONCLUSION The HAQ-DI showed good construct validity, internal consistency and reliability, whereas its content validity and responsiveness were limited. We recommend updating the items of the HAQ-DI in future research focusing on functional limitations in GOA. TRIAL REGISTRATION Dutch Trial Register NTR2137, http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=2137.