Validation of a Prediction Model to Estimate Health Utilities Index Mark 3 Utility Scores from WOMAC Index Scores in Patients with Osteoarthritis of the Hip

Objective: To examine the validity of a newly developed prediction model translating osteoarthritis (OA)-specific health-related quality of life (HRQL) scores measured using the Western Ontario and McMaster Osteoarthritis Index (WOMAC) into generic utility-based HRQL scores measured using the Health Utilities Index Mark 3 (HUI3). Methods: Preintervention data from 145 patients with hip OA and complete WOMAC and HUI3 baseline assessments from the Alberta Hip Improvement Project study were used to validate three utility prediction models. These models were estimated using data from a previous study of knee OA patients. Predictive performance was assessed using the mean absolute prediction error (MAE) criterion and several other criteria. Results: The validation sample appeared healthier (on the basis of the HUI3 and WOMAC) than the subjects used toestimate the prediction models. Nevertheless, the validation sample outperformed the predictive performance of the model sample. The results from the validation sample support the conclusions from the original study in that the primary model identified during model development (a model using WOMAC subscales, their interactions, their square terms, age, OA duration, their square terms, and gender) performed better on the MAE criterion than competing models. Conclusion: These results support the external validity of the prediction model for the retrospective estimation of HUI3 utility scores for use in economic evaluation.