UNLABELLED: Effectiveness and limitations of a DBT-informed day-patient treatment for patients with borderline personality disorder Introduction: Borderline personality disorder, a highly prevalent personality disorder is associated with remarkable impairment and is considered one of the most challenging mental illnesses to treat. Dialectical Behavioral Therapy has been recommended by the American Psychiatric Association as a specific treatment for patients with borderline personality disorder. So far, little is known about its effectiveness in a day-patient setting.

METHODS: This retrospective longitudinal study investigates changes in symptom burden during an average of 11 weeks of DBT-informed treatment at discharge, and three months after discharge. The symptomatology of n = 178 patients with borderline personality disorder treated from 2009 to 2017 was investigated with established borderline-specific (BSL) and -unspecific questionnaires (BSI-18, BDI) at admission, discharge, and 3-months follow-up by calculating mixed models, effect sizes, and response rates.

RESULTS: 80 % of the patients completed the treatment regularly. In borderline-specific impairments, there were moderate and highly significant improvements with good effects and a response rate of 48 %. Approximately 20 % showed a symptom level equivalent to that of the general population. The strongest effect sizes of approximately .8 were obtained for general psychopathology, with a response rate of 66 %. Results remained stable at follow-up.

DISCUSSION: Similar effects to inpatient treatment with good acceptance and efficacy could be achieved. Effect sizes differed for borderline-specific and unspecific symptoms, suggesting that DBT has different effects on different symptom areas. When comparing responders and non-responders, outpatient psychotherapy appeared to have a positive impact on the therapeutic effect. Furthermore, the results suggest that changes in borderline personality disorder extend over a longer period of time, which may indicate the limitations of curative treatment.