“When is it late”? Optimal threshold for duration of untreated illness (DUI) to predict SSRI-treatment resistance in individuals with obsessive-compulsive disorder (OCD)

Background: Obsessive-compulsive disorder (OCD) is a chronic psychiatric condition in which delays to appropriate treatment-known as duration of untreated illness (DUI)-are common and clinically consequential. Although prolonged DUI has been associated with poor response, the specific time point beyond which treatment resistance becomes likely remains unclear. Methods: We analysed 220 adults with DSM-5 OCD consecutively recruited at the University of Turin OCD clinic (2015-2023). DUI was defined as the interval between onset of clinically significant symptoms and the initiation of an adequate selective serotonin reuptake inhibitor (SSRI) trial (moderate-to-high dose for ≥12 weeks). Response was defined as a ≥35 % reduction in Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) score and a Clinical Global Impression-Improvement (CGI-I) rating ≤2. Receiver-operating-characteristic (ROC) analysis identified the DUI cut-off predicting SSRI non-response. Results: Mean age was equal to 34.5 ± 12.4 years; mean DUI was 107.2 ± 116.7 months. Half of the sample (50.4 %) responded to first-line SSRIs. ROC analysis yielded area under the curve = 0.634 (p < 0.001). The optimal cut-off was 42 months, corresponding to a sensitivity of 70.1 % and a specificity of 53.9 %. Conclusions: A DUI exceeding about 3.5 years is associated with a substantially lower probability of SSRI response. These data suggest that a duration of untreated illness beyond forty-two months may predict reduced responsiveness to first-line SSRI therapy in OCD, though replication in larger, multicentric samples is warranted.