Effect of prior depression diagnosis on bipolar disorder outcomes: a retrospective cohort study using a medical claims database

Background Bipolar disorder often emerges from depressive episodes and is initially diagnosed as depression. This study aimed to explore the effects of a prior depression diagnosis on outcomes in patients diagnosed with bipolar disorder. Methods This cohort study analyzed data of patients aged 18–64 years who received a new bipolar disorder diagnosis in Japan, using medical claims data from January 2005 to October 2020 provided by JMDC, Inc. The index month was defined as the time of the bipolar diagnosis. The study assessed the incidence of psychiatric hospitalization, all-cause hospitalization, and mortality, stratified by the presence of a preceding depression diagnosis and its duration (≥ 1 or < 1 year). Hazard ratios (HRs) and p-values were estimated using Cox proportional hazards models, adjusted for potential confounders, and supported by log-rank tests. Results Of the 5,595 patients analyzed, 2,460 had a history of depression, with 1,049 experiencing it for over a year and 1,411 for less than a year. HRs for psychiatric hospitalization, all hospitalizations, and death in patients with a history of depression versus those without were 0.92 (95% CI = 0.78–1.08, p = 0.30), 0.87 (95% CI = 0.78–0.98, p = 0.017), and 0.61 (95% CI = 0.33–1.12, p = 0.11), respectively. In patients with preceding depression ≥ 1 year versus < 1 year, HRs were 0.89 (95% CI = 0.67–1.19, p = 0.43) for psychiatric hospitalization, 0.85 (95% CI = 0.71-1.00, p = 0.052) for all hospitalizations, and 0.25 (95% CI = 0.07–0.89, p = 0.03) for death. Conclusion A prior history and duration of depression may not elevate psychiatric hospitalization risk after bipolar disorder diagnosis, and might even correlate with reduced hospitalization and mortality rates.