Internal Family Systems Therapy for Borderline Personality Disorder

Severity of borderline personality disorder symptomatology - disorder-specific mental distress

This first indicator of the primary outcome will be assessed by the Borderline Symptom List (BSL-23, Bohus et al., 2009). This self-report measure consists of items that 1) are based on DSM diagnostic criteria as well as expert opinions and expert-by-experience input; and 2) displayed a high level of sensitivity to change and ability to discriminate individuals with versus without borderline personality disorder. Scale items cover areas such as affective instability, emotional pain, identity disturbance, interpersonal sensitivity, cognitive-perceptual distress, impulsivity-related subjective distress, and suicidal ideation. Total scores range from 0 to 92 with higher scores indicating greater severity of borderline personality disorder symptoms.

Severity of borderline personality disorder symptomatology - dysfunctional behaviors

This second indicator of the primary outcome will be assessed by the supplementary scale of the Borderline Symptom List (BSL). This 11-item scale (BSL-Supplement; Bohus et al., 2007) assesses the extent of dysfunctional behaviours associated with borderline personality disorder such as self-injurious behaviour, suicidality, binge and purge behaviours, impulsivity, hostile outbursts, and sexual promiscuity. Total scores range from 0 to 44 with higher scores reflecting greater frequency or severity of borderline-related behaviors.

Emotion dysregulation

Emotion dysregulation will be assessed using the abbreviated, 18-item version of the Difficulties in Emotion Regulation Scale (DERS-18). The DERS-18 is a self-report measure that captures key dimensions of emotion dysregulation, including lack of emotional clarity, nonacceptance of emotional responses, difficulties engaging in goal-directed behavior, impulse control difficulties, and limited access to effective regulation strategies. Total scores range from 18 to 90, with higher scores indicating greater emotion dysregulation. Despite its brevity, the DERS-18 demonstrates strong psychometric properties and retains coverage of the core domains assessed by the original full-length version (Gratz \& Roemer, 2004).

Addictive behaviors

Addictive behaviours will be assessed using the Generalized Version of the Screener for Substance and Behavioural Addictions (SSBA-G). The SSBA-G is an 8-item, transdiagnostic self-report tool that screens for engagement in and perceived problematic use of a broad range of substances and behaviours (Konkolÿ Thege et al., 2023). Total scores range from 8 to 56, with higher scores indicating more severe addiction-related problems. This tool was selected over disorder- or substance-specific instruments due to its comprehensive scope, brevity, and ability to capture cross-addiction tendencies often observed in the target population.

Interpersonal functioning

Interpersonal functioning will be assessed by the Experience in Personal Social Systems Questionnaire (Hunger et al., 2017). This 12-item scale covers four dimensions in relation to 1) belonging (a sense of being part of a social system); 2) autonomy (ability to demark boundaries based on the belief that the renegotiation of roles, rights, and responsibilities is possible); 3) accord (acceptance of one's social system the way as it is); and 4) confidence (trust in the individual's and their social system's ability to cope with emerging difficulties). Total scores range from 12 to 72, with higher scores indicating better interpersonal functioning. This scale demonstrated good psychometric properties and sensitivity to change in intervention studies. This measure was chosen due to its positive approach to interpersonal functioning (instead of a focus on pathology) and brevity compared to other measures assessing interpersonal functioning.

Shame and guilt

The Guilt and Shame Questionnaire (GSQ-8) is a brief, validated self-report instrument designed to assess the frequency of guilt and shame experiences in the past month (Hoppen et al., 2022). The scale consists of 8 items-four assessing guilt and four assessing shame-rated on a Likert scale. It has demonstrated good internal consistency and a stable two-factor structure across clinical and non-clinical samples. Total scores range from 0 to 32, with higher scores indicating greater levels of guilt and shame. The GSQ-8 is sensitive to changes in self-conscious emotions and provides a concise, practical tool for repeated use in clinical research.

Intervention feasibility

It will be captured how many participants initiated versus completed the intervention (defined as attending ≥70% of the 50 therapy sessions offered). Higher proportion of completers indicates better feasibility.

Intervention tolerability

Rating \[from 0 (no distress) to 10 (extreme distress)\] in relation to the question: "How difficult or distressing did you find participating in the therapy you received as part of this study?" Higher scores are indicative of worse tolerability.

Intervention satisfaction

Rating \[from 0 (not at all satisfied) to 10 (extremely satisfied)\] in relation to the question: "Overall, how satisfied are you with the therapy you received as part of this study?" Higher scores are indicative of greater service user satisfaction.

Intervention relevance

Rating \[from 0 (not at all relevant) to 10 (extremely relevant)\] in relation to the question: "What do you think, how relevant this specific form of therapy (focus on internal parts and their relationship to each other and the Self) was to your particular mental health condition?" Higher scores are indicative of greater intervention relevance for the target population.