Psychotherapy for borderline personality disorder is often lengthy and resource-intensive. Mentalization-based therapy is an example of an evidence-based treatment that currently has empirical support as an 18-months outpatient program for borderline personality disorder. However, this duration is rarely available, and the long and costly treatment combined with a highly prevalent disorder result in insufficient access to evidence-based care.~The trial is an investigator-initiated, single-centre, assessor-blinded, randomized clinical superiority trial of short-term (20 weeks) compared to long-term (14 months) outpatient mentalization-based therapy for borderline personality disorder or subthreshold borderline personality disorder. Participants will be recruited from the Outpatient Clinic for Personality Disorders at Stolpegaard Psychotherapy Centre, Mental Health Services, Capital Region of Denmark. Participants will be assessed at trial intake using the Mini International Neuropsychiatric Interview and the Structured Clinical Interview for DSM-5 Personality Disorders. Participants will be included if they meet a minimum of four DSM-5 criteria for borderline personality disorder. Participants will be assessed blind to treatment allocation at baseline, and at 8, 16, and 24 months after randomization.~The primary outcome is severity of borderline symptomatology assessed using the Zanarini Rating Scale for Borderline Personality Disorder interview. Secondary outcomes include self-harm incidents, functional impairment (Work and Social Adjustment Scale), quality of life (Short-Form Health Survey), and global functioning (Global Assessment of Functioning scale). Psychiatric symptoms (Symptom Checklist 90) will be included as an exploratory outcome. Measures of personality functioning, attachment, group alliance, borderline symptoms and mentalization skills will be included as predictor and mediator variables.
The study will evaluate the benefitial and harmful effects of short-term (20 weeks) compared to long-term (14 months) mentalization-based therapy for outpatients with subthreshold or diagnosed borderline personality disorder.