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

General symptom severity as measured using the Brief Symptom Inventory (BSI-53). Items are scored on a 0 to 4 scale. The Global Severity Index (GSI) represents overall distress, with higher scores indicating greater severity of symptoms. The primary estimate of interest is the change in GSI score from the 3-year follow-up assessment to the long-term follow-up assessment conducted 10-14 years after treatment start. Repeated measures collected between baseline and 3-year follow-up will be incorporated in the analyses to account for prior trajectories.

Interpersonal problems are measured with the Inventory of Interpersonal Problems (IIP-64). Items are rated on a 0 to 4 scale, with higher scores indicating more interpersonal problems across eight domains (for example, nonassertive, socially inhibited). The primary estimate of interest is the change in IIP total score from the 3-year follow-up assessment to the long-term follow-up assessment conducted 10-14 years after treatment start. Repeated measures collected between baseline and 3-year follow-up will be incorporated in the analyses to account for prior trajectories.

Quality of life is measured using the EuroQol-5D-3L (EQ-5D-3L). Each of five dimensions has three response levels. Higher values on the index score reflect better health-related quality of life. The primary estimate of interest is the change in the index score from the 3-year follow-up assessment to the long-term follow-up assessment conducted 10-14 years after treatment start. Repeated measures collected between baseline and 3-year follow-up will be incorporated in the analyses to account for prior trajectories.

Reflective functioning is measured using the Reflective Functioning Questionnaire-8 (RFQ-8). Scores reflect uncertainty or certainty regarding mental states. The Uncertainty subscale has shown most sensitivity to change and evidence on the psychometric qualities concerning the Certainty subscale has shown mixed results, therefore we will focus on the subscale Uncertainty as outcome measure for the domain of reflective functioning. The primary estimate of interest is the change in Uncertainty subscale score from the 3-year follow-up assessment to the long-term follow-up assessment conducted 10-14 years after treatment start. Repeated measures collected between baseline and 3-year follow-up will be incorporated in the analyses to account for prior trajectories.

Personality functioning is assessed using the Severity Indices of Personality Problems-Short Form (SIPP-SF). Higher scores indicate better adaptive personality functioning across five core domains. The primary estimate of interest is the change in the five subscale scores from the 3-year follow-up assessment to the long-term follow-up assessment conducted 10-14 years after treatment start. Repeated measures collected between baseline and 3-year follow-up will be incorporated in the analyses to account for prior trajectories.

Impairments in personality functioning are assessed using the Level of Personality Functioning Scale-Brief Form 2.0 (LPFS-BF 2.0). Higher scores reflect greater impairment in self- and interpersonal functioning. No direct comparison will be made with previous assessments, as this measure was not included in the original trial. Scores at long-term follow-up will be interpreted using available normative scores to determine level of functioning.

Sense of belonging is measured by means of the PROMIS Short Form v2.0 Social Isolation. Participants indicate how often they feel left out, that people barely know them, isolated from others, and that people are around them but not with them in the past month on a Likert-type scale from 1 (never) to 5 (always). No direct comparison will be made with previous assessments, as this measure was not included in the original trial. Scores at long-term follow-up will be interpreted using available normative scores to determine level of functioning.

Life satisfaction is assessed by means of the Quality of Life Enjoyment and Satisfaction Questionnaire Short-Form (Q-LES-Q-SF). The Q-LES-Q-SF is a 16-item self-report questionnaire that measures the degree of enjoyment and satisfaction experienced in various domains of daily functioning, including work, social relationships, household activities, economic status and overall wellbeing on a 5 point likert scale ranging from 1 (very poor) to very good (5). No direct comparison will be made with previous assessments, as this measure was not included in the original trial. Scores at long-term follow-up will be interpreted using available normative scores to determine level of functioning.

Psychosocial functioning is assessed means of the 12-item WHO Disability Assessment Schedule (WHODAS 2.0). The WHODAS measures disability and functional impairment across six domains: cognition, mobility, self-care, interpersonal relationships, life activities, and participation in society. No direct comparison will be made with previous assessments, as this measure was not included in the original trial. Scores at long-term follow-up will be interpreted using available normative scores to determine level of functioning.

BPD diagnosis is assessed using the BPD section of the Dutch version of the SCID-II. Diagnostic status is operationalized as the dimensional score on BPD criteria, the number of criteria met, and the presence or absence of a BPD diagnosis. Current diagnostic status at long-term follow-up (10-14 years post-treatment) will be compared with previously collected data from the 3-year follow-up.

Health service use and productivity losses are assessed using the TiC-P questionnaire. Higher estimates reflect greater service utilization and higher direct and indirect costs. Health care utilization during the year prior to the long-term follow-up assessment will be compared to utilization during year prior to the baseline, and the year prior to the 3-year follow-up assessment.

Participants' experiences of MBT and its impact on symptoms, daily life functioning, and relationships, and views on treatment intensity, assessed through a semi-structured interview at long-term follow-up.