"When is it late"? Optimal threshold for duration of untreated illness (DUI) to predict SSRI-treatment resistance in...

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.

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.

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 %.

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.