Long Term Follow-up of Mentalization-Based Treatment: Exploring Variability in Outcomes, Long-term Value and Experiences 10+ Year After Receiving MBT Day Hospital or MBT Intensive Outpatient

This study is a long-term follow-up study of a previous multicenter randomized controlled trial, in which n=114 participants were included. This RCT compared the effectiveness of two intensities of mentalisation-based treatment (MBT) for individuals with borderline personality disorder (BPD).

The goal of this study is to learn how people who received MBT in the past for BPD are doing more than 10 years later. MBT is a type of psychotherapy that helps people understand and manage their thoughts and feelings, and supports improvements in identity and relationships, with the aim of improving daily life functioning.

The main questions this study aims to answer are:

1. How are people who received MBT in the past functioning more than 10 years later, and how is their use of mental health care and associated costs at this point in their lives? 2. Do long-term outcomes differ between people who received day-hospital MBT and people who received intensive outpatient MBT, and are these outcomes influenced by how much time has passed since treatment ended or by clinical characteristics from the past, such as symptom severity, trauma history, or level of mentalizing? 3. How do people who received MBT experience its impact on their symptoms, daily life, and relationships both during treatment and in the years afterward, including the impact of treatment intensity?

Participants will:

* Fill in online questionnaires about symptoms, relationships, health, and daily functioning (about 60 minutes). * Take part in a short interview to check whether BPD symptoms are still present (about 20 minutes). * A smaller group will be invited for a longer semi-structured qualitative interview (about 60 minutes) to talk about their personal experiences with MBT and what has impacted their life after treatment.

There are no new treatments in this study. All participants completed MBT many years ago. Participation happens online or in person based on personal preference.