Comparison of generic and disease-specific measures of quality of life in first-episode psychosis.

Background: Quality of life (QOL) is now recognised as an important measure of outcome that could potentially influence clinical decision-making for those with a first-episode psychosis (FEP). A number of QOL instruments are available however; many differ in their conceptual orientation which may have serious implications for the outcome of QOL studies, interpretation of findings and clinical utility. We aimed to compare two commonly used tools representing both generic and disease-specific constructs to examine whether both tools appraise the same underlying QOL traits and also whether disease-specific tools retain their psychometric properties when used in FEP groups.

Methods: We assessed 159 consecutive individuals presenting with FEP in a defined catchment area with two commonly used QOL tools and examined the findings using the multi-trait multi-method matrix.

Results: Similarly named domains of QOL between both tools (Psychological Wellbeing, Physical Health, Social Relations) showed good convergent validity using confirmatory factor analysis. However, discriminant validity was not established given that domains loading onto their indicated latent factors were more strongly correlated with their non-corresponding latent factors.

Conclusions: A major consideration in undertaking the present study was to assess the extent to which the outcome of QOL studies in FEP were valid and that systematic error did not provide another plausible explanation for findings. Establishing convergent validity demonstrates that either tool could be used satisfactorily to measure the QOL construct identified however; we did not establish discriminant validity. Doing so would have demonstrated that QOL domains are substantively different in that they contain some unique piece of information determining clinical utility. These findings are important for our understanding of multi-dimensional models of QOL.