Mapping From The Womac To The EQ-5D-5L Questionnaire: Comparison Of Different Methodologies

OBJECTIVES: Hip or knee osteoarthritis (OA) affects very negatively the health-related quality of life (HRQoL). Consequently, studies of treatment efficiency, typically conducted using health utilities are of great interest. One of the most widely used generic instruments to derive utilities is the EQ-5D-5L. However, in clinical practice, the use of specific HRQoL questionnaires is more frequent. Our objective was to develop mapping functions to estimate the utility index from the WOMAC questionnaire. METHODS: Prospective observational study, including 748 patients from Spain with hip or knee OA who completed the EQ-5D-5L and WOMAC, of whom 626 responded to the 6-months follow-up. Using the baseline data we derived the mapping functions from two WOMAC dimensions: pain (P) and function (F). GAM models and bootstrap were used to determine the optimal relationship grade and combinations of the WOMAC domains, and then, two strategies were used for the modelling: linear and beta regression. To select the best model the AIC was used. These functions were validated in the follow-up data using MAE and RMSE. RESULTS: The mean EQ-5D-5L index was 0.533 (SD=0.223, range=-0.416 to 1). The best combination of WOMAC domains were: P3+F and P-F+F. Both linear and beta models obtained similar AIC values for both combinations, although the validation of these functions in the follow-up sample showed slightly lower MAE and RMSE for the P-F+F. Further, the results of the beta model were not better than the linear model. Based on linear model the function was: EQ-5D-5L=0.9525-0.000056-P-F-0.0051-F (R2=0.618, AIC=-455.39, MAE=0.135, RMSE=0.178). CONCLUSIONS: As far as we know, this is the first mapping function from the WOMAC to the Spanish EQ-5D-5L in patients with hip or knee OA. It could be very useful for clinicians or researchers when cost-effectiveness studies are needed, and generic HRQoL instruments to derive utility indexes are not available.