Borderline personality disorder does not predict treatment outcome in a partial hospital program independent of internalizing and harmful substance use dimensions.

Borderline personality disorder (BPD) diagnoses reliably predict worse mental health treatment outcomes. It is unknown whether multidimensional models of psychopathology, increasingly viewed as viable alternatives to traditional diagnostic systems, can match BPD's predictive power in clinical settings. In a sample of 2,625 partial hospital patients, we compared BPD with internalizing and harmful substance use dimensions as predictors of treatment success, defined by improvement in symptom severity, functional impairment, and quality of life. On a bivariate level, BPD features were moderately related to posttreatment functional impairment (r = .28) and quality of life (r = -.24), but not self-rated symptom improvement (r = .04). When adjusting for internalizing and harmful substance use, however, BPD features had very little unique predictive power. Instead, the internalizing spectrum was the strongest, most consistent predictor of symptom and functional improvement. This pattern of effects suggests that broad dimensions of psychopathology, particularly internalizing, capture much of BPD's prognostic utility in a partial hospital setting. We advise more attention to how the traditional BPD diagnosis compares to broad psychopathology dimensions in terms of clinical utility. This study's data, analysis code, and materials are posted at https://osf.io/wpy7e. (PsycInfo Database Record (c) 2026 APA, all rights reserved).