Objective: Prior studies have reported that major depressive episodes (MDEs) are associated with elevated mortality. However, the association has not always persisted after adjustment for other mortality risk factors. In our study, we examine this issue using data from a longitudinal Canadian study (the National Population Health Survey [NPHS]). The NPHS included a more comprehensive set of mortality determinants than prior studies, allowing a more comprehensive assessment of the effect of MDEs on mortality.
Methods: The NPHS began data collection in 1994 and follow-up data were available to 2006 at the time of this analysis. The NPHS assessed depression using a short-form version of the Composite International Diagnostic Interview. Mortality was assessed as part of the cohort's follow-up, including linkage to vital statistics data.
Results: During follow-up, 2019 deaths occurred in the eligible part of the NPHS cohort. Consistent with prior studies, MDEs were strongly predictive of mortality when basic adjustments were made for age and sex (hazard ratio [HR] 2.0, 95% CI 1.4 to 2.9). The association disappeared with adjustment for other variables that predict mortality risk (HR 1.1, 95% CI 0.7 to 1.7).
Conclusions: MDE is a strong predictor of mortality in the general population. This analysis failed to identify an independent effect of MDE when adjustments were made for other risk factors. However, the lack of a strong independent effect on mortality does not preclude an etiologic impact of MDE. MDE itself is intertwined with health-related changes that predict mortality and its impact may be mediated by these variables.