Mapping functions for the PHQ-9 and GAD-7 to generate EQ-5D-3L for economic evaluation

Generic preferenced-based measures, such as EQ-5D-3L, that are used to estimate quality adjusted life years (QALYs) for economic evaluation are not always available in clinical trials. Predicting EQ-5D-3L values from the commonly used Patient Health Questionnaire 9 (PHQ-9) and Generalised Anxiety Disorder-7 (GAD-7) would allow estimation of QALYs from such trials. The aim was to provide mapping functions to estimate EQ-5D-3L from PHQ-9 and GAD-7 to facilitate economic evaluation. Data was drawn from four trials of patients with symptoms of depression testing collaborative care or computerised cognitive behavioural therapy. Patients completed PHQ-9, GAD-7, and EQ-5D-3L at different timepoints. Mapping was undertaken using adjusted limited dependent variable mixture models (ALDVMM), ordinary least squares (OLS), and Tobit models based on PHQ-9, GAD-7 scores or questions, and age to predict EQ-5D-3L utilities. Models were selected based on mean error (ME), mean absolute error (MAE), root mean squared error (RMSE), model goodness of fit, and visual inspection of the predictions. There were 5583 and 3942 observations for EQ-5D-3L combined with PHQ-9 and GAD-7 respectively. ALDVMM models had low ME ( ≤|0.0018|) and MAE ranging from 0.189 to 0.192, while RMSE was from 0.251 to 0.254 and had better predictions than OLS and Tobit models. ALDVMM models with four components based on PHQ-9 and GAD-7 scores are recommended for estimating EQ-5D-3L utilities. Recommended mapping functions provide users with an approach to estimate EQ-5D-3L utilities for economic evaluation using PHQ-9, GAD-7, or both scores where they have been used together.