Differentiating bipolar disorder from borderline personality disorder: Diagnostic accuracy of the difficulty in emotion regulation scale and personality inventory for DSM-5

•Personality traits differentiate bipolar disorder (BD) from borderline personality disorder (BPD) patients.•PID-5 traits of emotional lability, anxiousness, separation insecurity, hostility, depressivity, impulsivity, and risk taking provided good specificity (SP = 0.79) and sensitivity (SN = 0.77).•Self-report measure of emotion dysregulation was marginally successful in differentiating BD from BPD patients.•Tools that make complex differential diagnosis more accurate have important downstream benefit to patients. Confusion abounds when differentiating the diagnoses of bipolar disorder (BD) from borderline personality disorder (BPD). This study explored the relative clinical utility of affective instability and self-report personality trait measures for accurate identification of BD and BPD. Receiver operator characteristics and diagnostic efficiency statistics were calculated to ascertain the relative diagnostic efficiency of self-report measures. Inpatients with research-confirmed diagnoses of BD (n = 341) or BPD (n = 381) completed the Difficulty in Emotion Regulation Scale (DERS) and Personality Inventory for DSM-5 (PID-5). The total score for DERS evidenced relatively poor accuracy for differentiating the disorders (AUC = 0.72, SE = 0.02, p