Background: Cost-utility analyses using formulas to convert depression-free days (DFDs) to utility-weighted scores are increasingly common. These formulas are based on linear extrapolation of data documenting the correlation between depression symptom severity and generic health-related quality of life.
Objective: We sought to examine the validity of formulas converting DFDs to utility weights.
Methods: We undertook an observational study with data collection at baseline, 1 week and 1, 3, 6, 9, and 12 months on 77 subjects (42 inpatient, 35 outpatient) diagnosed with current major depression. Subjects were divided into treatment response categories based on changes in depression severity. Depression severity measures used were the Hamilton Rating Scale for Depression (HAM-17) and Beck Depression Inventory (BDI) and the health-related quality of life measure was the self-administered Quality of Well-Being scale (QWB-SA). DFD calculations were based on depression severity scores and converted to utility weights using available formulas. Utility-weighted data collected over the course of 1 year were used to estimate quality-adjusted life years (QALYs).
Results: QALYs estimated from the QWB-SA were significantly lower than those based on utility-weighted DFD calculations but the incremental QALYs were not significantly different. Using a slightly lower utility-weighted conversion factor for the BDI or a larger BDI severity range to calculate DFDs resulted in a better fit compared with the QWB-SA.
Conclusions: Our results support the validity of the existing HAM-17 utility-weighted formula and suggest modifications for the BDI formula. If generic health-related quality of life measures are not available for conducting cost-utility analyses of depression interventions then the existing HAM-17 and modified BDI formulas appear to be reasonable alternatives.