Furukawa et al 1 posed the question: how can we estimate quality-adjusted life years (QALYs) based on Patient Health Questionnaire-9 (PHQ-9) scores? They recommend equipercentile linking analysis between the depression severity PHQ-9 and preference-based EQ-5D three-level version (EQ-5D-3L; UK value set), the latter used to estimate utility data for QALYs.

Furukawa et al 1 refer to the process of ‘cross-walking’, whereby the practice of fitting a statistical model to health utility data has been referred to as ‘mapping’ and 'cross-walking’.2 Furukawa et al 1 reference two mapping-related papers (their references 7 and 9); however, their analysis seems to have missed rigorous mapping methodology and previous studies which have used these mapping processes, alongside other conceptual considerations when wanting to ‘cross-walk’/‘map’ from a non-preference-based (often condition-specific) measure such as the PHQ-9 to the preference-based EQ-5D-3L. …