Measurement properties of the Disability Rating Index in patients undergoing hip replacement.

OBJECTIVE The aim of this study was to establish and validate the measurement properties of the Disability Rating Index (DRI) in a population of adults undergoing hip replacement. METHODS One hundred and twenty-six adults participating in a randomized controlled trial completed the Oxford Hip Score, Harris Hip Score, DRI and EuroQol Group-Five Dimensions (EQ-5D) questionnaires at four time points. The structural validity of the DRI was assessed using principal component analysis. Cronbach's α was used to determine the internal consistency and scale reliability was also assessed. Correlation between the DRI and the other functional and health-related quality of life scales was used to check criterion validity. DRI responsiveness was estimated and the interpretability of the scale was also assessed by checking for edge effects. RESULTS Results of analyses showed that the DRI was internally consistent (Cronbach's α = 0.92), had good association with both function-specific and general health-related quality of life scores and was sensitive to change (smallest detectable change = 2.7). No evidence of edge effects was found. Furthermore, structural assessment of the DRI revealed two novel subscales representing simple tasks and difficult tasks. CONCLUSIONS The DRI is structurally valid, responsive and concurs with functional assessment in adults undergoing hip replacement.