A 35-year-old man with a 10-year history of OCD (intrusive violent thoughts, checking compulsions) is started on sertraline. After 8 weeks at 200 mg daily, he reports minimal symptom reduction. Which investigation should be performed next to guide further management?

## Assessment of SSRI Non-Response in OCD **Key Point:** Y-BOCS is the primary investigation to objectively quantify treatment response and identify partial or non-response, guiding escalation or augmentation strategies. ### Why Y-BOCS Is Essential for Treatment Monitoring 1. **Objective response criteria**: A ≥25% reduction in Y-BOCS score from baseline defines treatment response; <25% reduction indicates non-response. 2. **Severity reassessment**: Allows comparison of baseline vs. current severity to determine if dose escalation, switching SSRIs, or augmentation is needed. 3. **Clinical decision-making**: Guides next steps (e.g., increase dose, switch to fluoxetine or paroxetine, add antipsychotic augmentation, refer for CBT/ERP). 4. **Standardized measurement**: Eliminates subjective bias in assessing whether the patient truly has inadequate response. **High-Yield:** Y-BOCS at 8 weeks post-initiation is the *standard investigation* to determine if the SSRI dose is adequate or if treatment modification is required. ### Treatment Response Pathway in OCD ```mermaid flowchart TD A[SSRI initiated at therapeutic dose]:::action --> B[Reassess at 8-12 weeks with Y-BOCS]:::decision B -->|≥25% reduction in Y-BOCS| C[Responder: Continue current dose]:::outcome B -->|<25% reduction in Y-BOCS| D{Non-responder: Next step?}:::decision D -->|Option 1| E[Increase SSRI dose]:::action D -->|Option 2| F[Switch to different SSRI]:::action D -->|Option 3| G[Add augmentation: Antipsychotic or CBT]:::action E --> H[Reassess with Y-BOCS at 12 weeks]:::decision F --> H G --> H ``` ### Why Other Investigations Are Inappropriate | Investigation | Rationale for Non-Use in This Context | |---|---| | **Serum sertraline levels** | Therapeutic drug monitoring is not standard for SSRIs in OCD; levels do not correlate reliably with clinical response | | **EEG** | No indication; seizure disorder is not a differential diagnosis here; EEG does not guide OCD management | | **PET scan** | Research tool only; not used for routine clinical assessment of treatment response | **Clinical Pearl:** The absence of objective Y-BOCS measurement at 8 weeks is a common clinical error. Many patients are kept on suboptimal doses or switched unnecessarily without quantified baseline-to-current comparison. Always obtain Y-BOCS before and after treatment changes. **Mnemonic:** **Y-BOCS at 8 weeks = Your Best Outcome Check Schedule** — measure severity at baseline and 8 weeks to guide SSRI optimization.