Supplementary Material for: Exploring the Black Box: What Happens During Brief Concentrated Exposure and Response Prevention for Obsessive-Compulsive Disorder?

Background: Brief concentrated exposure and response prevention (cERP) has shown promise as an efficacious treatment for obsessive-compulsive disorder (OCD). with higher response and remission rates compared to the first-line treatment. However, the mechanisms driving this success remain unclear.
Methods: This longitudinal study included 56 patients with OCD who underwent cERP (Bergen four-day treatment; B4DT). Treatment mechanisms (e.g., willingness to engage in ERP, confidence in future self-guided ERP, leaning into anxiety) were assessed before and after each of the four treatment days by the Pre- and Post-Session Questionnaire (PPSQ-cERP). Changes in the PPSQ-cERP were used to predict treatment response assessed from baseline to post-treatment and three-month follow-up.
Results: All variables assessed by the PPSQ-cERP showed improvement throughout the cERP, as calculated using linear mixed models. Several variables, including willingness to engage in ERP and self-efficacy, improved after day 3, with small to medium effects (0.34–0.70). Confidence in future self-guided ERP improved on day 4 (the day on which it was addressed), with a small effect (0.31). Leaning into anxiety during ERP tasks on day 3 was identified as a predictor of OCD symptom improvement ( = 0.516, p = .050) by lasso regression, while group cohesion reached trend level ( = 0.239, p = .081).
Conclusion: This study highlights day-specific effects across all treatment mechanism variables. Notably, leaning into anxiety during ERP tasks was a key predictor of symptom improvement, offering new insights into refining OCD treatment strategies and enhancing clinical outcomes.