A 38-year-old woman presents with persistent low mood, anhedonia, and sleep disturbance for 3 weeks. She denies psychotic symptoms. On examination, she appears psychomotor retarded. Which investigation is most appropriate to establish the diagnosis of Major Depressive Disorder?

Explanation

## Diagnosis of Major Depressive Disorder **Key Point:** Major Depressive Disorder is a clinical diagnosis based on DSM-5 criteria. No biological marker or investigation is required or sufficient for diagnosis. ### DSM-5 Diagnostic Criteria MDD requires: - ≥5 symptoms present for ≥2 weeks - Symptoms include depressed mood OR anhedonia (core symptoms) - Functional impairment - Exclusion of organic causes and bipolar disorder A structured clinical interview using DSM-5 criteria is the gold standard and most appropriate investigation for diagnosis. ### Why Other Investigations Are Not Diagnostic | Investigation | Role | Limitation | |---|---|---| | **DST** | Research tool for HPA axis dysfunction | Not diagnostic; poor sensitivity/specificity | | **fMRI** | Research; may show prefrontal/limbic changes | Not clinically indicated for diagnosis | | **Polysomnography** | Assesses sleep architecture | Supports sleep disturbance but not diagnostic | **High-Yield:** MDD diagnosis is **clinical**—based on history, symptoms, and functional impact. No blood test, imaging, or neuroendocrine test diagnoses MDD. **Clinical Pearl:** Always rule out organic causes (thyroid dysfunction, vitamin B12 deficiency, neurological disease) before attributing symptoms to primary MDD.