Which of the following is the most sensitive biochemical test for diagnosis of pheochromocytoma?

Explanation

## Biochemical Diagnosis of Pheochromocytoma **Key Point:** 24-hour urinary metanephrines (or plasma free metanephrines) is the gold standard screening test for pheochromocytoma due to superior sensitivity and specificity. ### Why Urinary Metanephrines? 1. **Sensitivity:** >96% — detects even small, non-secreting, or intermittently secreting tumours 2. **Specificity:** >85% — minimal false positives 3. **Stability:** Metanephrines are stable metabolites; less affected by stress, medications, or timing of collection compared to catecholamines 4. **Continuous production:** Reflects total catecholamine metabolism over 24 hours, capturing episodic secretion ### Comparison of Diagnostic Tests | Test | Sensitivity | Specificity | Advantages | Limitations | |------|-------------|-------------|-----------|-------------| | **24-h urine metanephrines** | >96% | >85% | Gold standard, stable, captures episodic secretion | Requires 24-h collection | | **Plasma free metanephrines** | >96% | >89% | Single blood draw, high sensitivity | Affected by posture, stress; requires supine collection | | **Plasma catecholamines** | 60–70% | Variable | Rapid result | Stress-sensitive, poor specificity, affected by medications | | **Urinary VMA** | 60–70% | 80% | Older test | Lower sensitivity, dietary interference (vanillates) | | **Chromogranin A** | 50–80% | 70% | Useful in MEN syndromes | Non-specific, elevated in other neuroendocrine tumours | **High-Yield:** If either 24-h urine metanephrines OR plasma free metanephrines is elevated, proceed directly to imaging (CT/MRI abdomen ± MIBG scan) without repeating biochemical tests. **Clinical Pearl:** Medications (decongestants, tricyclic antidepressants, sympathomimetics) and stress can falsely elevate catecholamines but NOT metanephrines — another reason metanephrines are preferred. **Tip:** In NEET PG, if a question asks for the "most sensitive" or "gold standard" biochemical test for pheochromocytoma, the answer is always **24-hour urinary metanephrines** or **plasma free metanephrines**.