Estimating QALYs in adults with cerebral palsy: mapping the San Martin scale to the EQ-5D-5L for economic evaluation

Responses on health-related quality of life measured by disease-specific instruments can be mapped onto the EQ-5D to estimate utility values for economic evaluation. San Martin´s Quality of Life Scale (St. MQoL-S) is a preferred measure to obtain health outcomes in adults with cerebral palsy. Nevertheless, it lacks a preference-based health utility score for estimating quality-adjusted life years (QALYs).

To develop algorithms for mapping from the St. MQoL-S to allow future prediction of the EQ-5D-5L, in adults with cerebral palsy, when utility data have not been collected.

Direct mapping models were developed using ordinary least squares, a generalized linear model, and Tobit regression analysis to estimate EQ-5D-5L utilities, with St. MQoL-S total and domain scores as explanatory variables, in a cross-sectional study of adults with cerebral palsy in Spain. Goodness-of-fit was assessed using mean absolute error (MAE) and root mean square error (RMSE). Repeated k-fold cross-validation was employed to select the optimal mapping model demonstrating superior predictive performance.

The best-performing model for predicting EQ-5D-5L utilities, includes the St. MQoL-S total scores, age, gender, and types of cerebral palsy as explanatory variables in a stepwise ordinary least squares regression, making it the most robust model for use as a mapping algorithm with external data.

This is the first study to present mapping algorithms between the St. MQoL-S and EQ-5D-5L. The mapping functions preferred in this study seem adequate for estimating the utilities of the EQ-5D-5L for economic evaluation and to obtain QALYs in adults with cerebral palsy.