Bipolar disorder and borderline personality disorder comorbidity: an 8-year population-based retrospective study

Comorbidity between bipolar disorder (BD) and borderline personality disorder (BPD) is associated with greater clinical severity, yet its impact in inpatient settings remains insufficiently characterized. This study aimed to evaluate the association between comorbid BPD, clinical characteristics and inpatient outcomes among BD hospitalizations.

A population-based retrospective study was conducted using a nationwide administrative database of public general and psychiatric hospital admissions in mainland Portugal over an 8-year period. Hospitalizations involving adults with a primary or secondary BD diagnosis were identified. Comorbid BPD was identified using ICD-9-CM code 301.83. Sociodemographic characteristics, psychiatric comorbidities, and hospitalization outcomes were compared between hospitalizations with and without BPD. Multivariable regression models adjusted for age, sex, and Charlson Comorbidity Index were applied.

A total of 32,610 hospitalizations were identified, of which 342 had comorbid BPD. Compared with BD-only admissions, BD + BPD hospitalizations involved younger patients, a higher proportion of women, more frequent urgent admissions, depressive and mixed episodes, and greater psychiatric comorbidity. Comorbid BPD was associated with higher odds of discharge against medical advice (aOR = 1.59; 95%CI: 1.02 to 2.47), suicidal ideation or self-harm (aOR = 3.60; 95%CI: 2.79 to 4.65), alcohol-related disorders (aOR = 3.31; 95%CI: 2.44 to 4.47), illicit drug-related disorders (aOR = 3.84; 95%CI: 2.87 to 5.13), and 1-year readmission (aOR = 1.34; 95%CI: 1.07 to 1.69), but not with 30-day readmission.

Among BD hospitalizations, those with comorbid BPD were characterized by greater psychiatric complexity, elevated suicide-related morbidity, and increased longer-term hospital utilization, highlighting the importance of improved recognition and targeted inpatient and post-discharge management.