OBJECTIVES: Depression is one of the most disabling mental disorders causing a significant decrease in health-related quality of life (HRQoL). Therefore, studies of treatment efficiency are of great interest. They are usually based on health utilities, being the EQ-5D-5L one of the most widely used instruments to derive these utilities. However, in clinical practice, the use of specifics questionnaires is more frequent. Our objective was to develop mapping functions to estimate the EQ-5D-5L utility index from the specific BDI-II questionnaire.

METHODS: A prospective observational study, including 418 patients from Spain with major depression, who completed the EQ-5D-5L and BDI-II questionnaires. Of these, 283 responded to 6-months follow-up. The baseline data was used to derive the mapping function from the BDI-II score. The GAM models were used to determine the optimal relationship grade between the utility index and the BDI-II score. Then, we used linear and beta regression for the modelling, and age and sex were also considered. To select the best model the AIC was used. These functions were validated with the follow-up data, and the fit was compared by the MAE and RMSE.

RESULTS: The mean EQ-5D-5L index was 0.562 (SD=0.269, range=-0.384 to 1). GAM models indicated no need of powers of the BDI-II score. Both linear and beta regression models obtained similar results for models with and without age and sex. However, the validation of these functions in the follow-up sample showed slightly lower MAE and RMSE values in the linear model. The function was: EQ-5D-5L=1.1390-0.0131·BDI-0.0036·Age+0.0180·Sex (Man) (R=0.351, AIC=-87.81; MAE=0.170, RMSE=0.215).

CONCLUSIONS: To the best of our knowledge, this is the first mapping function from the BDI-II to the Spanish EQ-5D-5L in patients with major depression. This function could be very useful if cost-effectiveness studies are needed and generic HRQoL questionnaires to derive utility indexes are not available.