A 32-year-old woman with a 2-year history of schizophrenia presents with persistent auditory hallucinations and negative symptoms despite adequate trials of two antipsychotics. Her compliance is confirmed. Which investigation is most appropriate to assess treatment resistance and guide further management?

## Investigation for Treatment-Resistant Schizophrenia (TRS) ### Definition of Treatment Resistance **Key Point:** Treatment-resistant schizophrenia is defined as failure to respond adequately to at least 2 adequate trials of antipsychotics (different classes, adequate dose, adequate duration ≥4–6 weeks) with documented compliance. ### Role of Therapeutic Drug Monitoring (TDM) **High-Yield:** When a patient on adequate antipsychotic doses shows poor response, TDM helps determine: 1. **Plasma concentration within therapeutic range?** → Suggests true resistance; consider clozapine. 2. **Subtherapeutic levels?** → Indicates malabsorption, drug interactions, or non-compliance despite reported adherence (covert non-compliance). 3. **Supratherapeutic levels?** → Toxicity; reduce dose or switch agent. **Clinical Pearl:** Serum antipsychotic levels vary widely (10–50-fold) between individuals due to genetic polymorphisms in CYP450 metabolism (especially CYP2D6, CYP3A4). A patient may appear "treatment-resistant" when they are actually a poor metabolizer with subtherapeutic levels, or a rapid metabolizer with inadequate exposure. ### Why TDM Is the Best Investigation | Aspect | TDM | PET | fMRI | EEG | |---|---|---|---|---| | **Cost** | Low | Very high | High | Low | | **Clinical utility** | High—guides dose/switch | Research only | Research only | Not indicated | | **Availability** | Widely available | Limited | Limited | Widely available | | **Actionable result** | Yes—adjust dose or switch | No—does not change management | No—does not change management | Not indicated for TRS | | **Evidence base** | Strong—standard of care | Experimental | Experimental | Not applicable | **Mnemonic: TDM for TRS** — **T**herapeutic **D**rug **M**onitoring for **T**reatment-**R**esistant **S**chizophrenia. ### Clinical Algorithm ```mermaid flowchart TD A[Patient on antipsychotic with poor response]:::outcome --> B{Compliance confirmed?}:::decision B -->|No| C[Improve adherence, psychoeducation]:::action B -->|Yes| D[Obtain serum antipsychotic levels]:::action D --> E{Levels therapeutic?}:::decision E -->|No - subtherapeutic| F[Increase dose or check absorption]:::action E -->|Yes - therapeutic| G[True treatment resistance]:::outcome G --> H[Consider clozapine]:::action H --> I[Gold standard for TRS]:::outcome ``` ### Why Not the Other Options? - **PET scan:** Research tool only; does not guide clinical management of TRS. No role in routine clinical practice. - **fMRI:** Experimental neuroimaging; not clinically validated for TRS assessment. Does not change treatment decisions. - **Repeat EEG:** Not indicated in TRS. EEG is for seizure disorders or altered consciousness, not for psychotic symptoms. **Key Point:** If TDM shows therapeutic levels, the diagnosis is **true treatment-resistant schizophrenia**, and **clozapine** is the gold standard—it is effective in 30–50% of TRS patients who fail conventional antipsychotics. [cite:Harrison 21e Ch 386; Maudsley Guidelines 2024]