Gender influence on the bipolar disorder inpatient length of stay in Sweden, 2005-2014: A register-based study

Background: The influence of gender on bipolar disorder is controversial and it is unclear if inpatient care differs between men and women. Here, we investigate for gender differences in the inpatient length of stay for Swedes admitted for bipolar disorder and explore other factors that could explain any observed association. Methods: Admission data were extracted from the Swedish National Patient Register and included all patients first admitted to a psychiatric inpatient unit with a bipolar disorder diagnosis, circa 2005-2014. Patients were then retrospectively followed for subsequent hospitalizations. Diagnostic subtypes were categorized by ICD-10 clusters: depressive, depressive with psychotic features, manic, manic with psychotic features, mixed, and other. Psychotropic therapies preceding the corresponding admissions were attained from the Prescribed Drug Register. Mixed-effects zero-truncated negative binomial regressions were employed to model the length of stay per admission. Results: Analysis included 39,653 admissions by 16,271 inpatients (60.0% women). Overall, when compared to men, women spent 7.5% (95% CI: 4.2-11.0%, p < 0.001) extra days hospitalized per admission. However, upon adjusting for candidate confounders, including the bipolar subtype, and selected comorbidities and psychotropics, the association weakened wherein women then spent 3.7% (95% CI: 0.1-6.9%, p = 0.028) extra days hospitalized per admission. Limitations: The integrity of register data can be variable and the adherence to outpatient dispensed psychotropics could not be validated. Conclusion: Although the influence of gender on the bipolar disorder inpatient length of stay is evident, other factors attenuate and better explain this crude observation.