Mortality-causing mechanisms and healthcare resource utilisation of treatment-resistant depression: A six-year population-based cohort study.

Background: Few studies investigated the mechanisms of treatment-resistant depression (TRD) leading to the worsened survival outcome, and economic evidence was mostly restricted to short follow-ups. We aimed to examine the association and potential mediators between TRD and all-cause mortality, and estimate a longer-term associated health resource utilisation pattern. Methods: This was a population-based cohort study using territory-wide electronic medical records in Hong Kong. Incident depression patients diagnosed in 2014 were followed up from the first diagnosis to death or December 2019 for TRD identification. We matched the TRD cohort 1:4 to the non-TRD cohort on propensity scores estimated by age, sex, history of physical disorders, and history of psychiatric conditions before depression diagnoses. Findings: 18% of incident patients developed TRD within six years of follow-up. Cox model showed that patients with TRD had 1⋅52-fold (95% CI: 1⋅14-2⋅02) greater risk of all-cause mortality, compared with non-TRD patients. Path analysis suggested that post-TRD psychiatric conditions significantly mediated 41⋅6% of mortality in patients with TRD (p=0.003). TRD was associated with 1⋅8-fold (95%CI: 1⋅63-2⋅00) higher healthcare costs compared to non-TRD patients over six years in negative binomial regression, with higher costs for both psychiatric and non-psychiatric services utilisation in all settings. Interpretation: Identifying patients with TRD and subsequent monitoring for post-TRD psychiatric diagnoses could be a way to reduce premature mortality. Multidisciplinary care involving both psychiatric and general medical professionals is also warranted to relieve the multifaceted impacts on healthcare resources and overall cost. Funding: Unconditional educational grant from Janssen.