PURPOSE: To evaluate the psychometric properties of the EQ-5D and the usefulness of this instrument in psychiatric practice as a measure of health-related quality of life (HRQoL) in a sample of young adults with first-episode psychosis. METHODS: The validity of individual questionnaire items is studied using an Item Response/Latent Trait Theory modeling approach. Sensitivity of response patterns on EQ-5D items to particular diagnostic subtypes of psychosis is investigated using a finite mixture modeling approach through latent class analysis. Finally, a structural equation modeling framework is used to study differential item functioning via a multigroup approach. RESULTS: Results suggest that the data closely correspond to the Rasch Rating Scale Model, and therefore that EQ-5D latent scores are equal interval measures. Despite comprising relatively few items, the instrument yields reliable measures of HRQoL for group comparisons and cost-effectiveness evaluation, but EQ-5D score is too imprecise for the assessment of HRQoL for clinical purposes at the individual level. A significant relationship was found between EQ-5D responses and type of psychosis due to inclusion of item anxiety/depression in EQ-5D. Two items (anxiety/depression, functioning in usual activities) showed an ethnicity bias. CONCLUSIONS: Psychometric evidence confirmed the EQ-5D to be a valid, interval measure that is scalable according to Rasch principles.
- Public Health, Environmental and Occupational Health