The change in symptoms necessary to be clinically relevant in obsessive-compulsive disorder (OCD) is currently unknown. In this study, we aimed to create an empirically validated threshold for clinical significance or minimal important difference (MID).

We analyzed individual participant data from short-term, double-blind, placebo-controlled registration trials of selective serotonin reuptake inhibitors in adult OCD patients. Data were collected from baseline to week 12. We used equipercentile linking to equate changes in the Clinical Global Impression (CGI) scale to changes in the Yale-Brown Obsessive-Compulsive Scale (YBOCS). We defined the MID as the YBOCS change linked to a CGI improvement of 3 (defined as “minimal improvement”).

We included 7 trials with a total of 1216 patients. The CGI-scores and YBOCS were moderately to highly correlated. The MID corresponded to 4.9 YBOCS points (95% CI 4.4–5.4) for the full sample, or a 24% YBOCS-decrease compared to baseline. The MID varied with baseline severity, being lower in the group with mild symptoms and higher in the group with severe symptoms.

By linking the YBOCS to the CGI-I, this is the first study to propose an MID in OCD trials. Having a clearly defined MID can guide future clinical research and help interpretation of efficacy of existing interventions. Our results are clinician-based; however, there is further need for patient-reported outcomes as anchor to the YBOCS.