A 38-year-old man presents with episodic headaches, profuse sweating, and palpitations for 3 months. Blood pressure is 165/105 mmHg at rest. Clinical suspicion for pheochromocytoma is high. Which investigation is the most appropriate initial confirmatory test?

Explanation

## Initial Biochemical Confirmation of Pheochromocytoma **Key Point:** Plasma free metanephrines is the single most sensitive and specific biochemical test for diagnosing pheochromocytoma and should be the first-line confirmatory investigation. ### Why Plasma Free Metanephrines? **High-Yield:** Plasma free metanephrines have >96% sensitivity and >89% specificity for pheochromocytoma. They are superior to 24-hour urine metanephrines because: - Metanephrines are the O-methylated metabolites of catecholamines, produced continuously by the tumor - Plasma levels are less affected by stress, posture, and medications compared to catecholamines - Single blood draw is more convenient than 24-hour urine collection - Results are available faster **Clinical Pearl:** The test should be performed with the patient supine for 30 minutes before blood draw to minimize stress-related elevation. Certain medications (decongestants, tricyclic antidepressants, some antipsychotics) must be discontinued 2 weeks prior if possible. ### Investigation Algorithm ```mermaid flowchart TD A[Clinical suspicion for pheochromocytoma]:::outcome --> B[Plasma free metanephrines]:::action B --> C{Normal?}:::decision C -->|Yes| D[Pheochromocytoma excluded]:::outcome C -->|No| E[Elevated metanephrines]:::outcome E --> F[Proceed to imaging]:::action F --> G[CT/MRI abdomen & pelvis]:::action G --> H{Tumor found?}:::decision H -->|Yes| I[Functional imaging: MIBG scintigraphy]:::action H -->|No| J[Consider extra-adrenal tumor]:::action ``` ### Comparison of Biochemical Tests | Test | Sensitivity | Specificity | Advantages | Disadvantages | |------|-------------|-------------|-----------|---------------| | Plasma free metanephrines | >96% | >89% | Most sensitive; single draw; less affected by stress | Requires proper patient positioning; some medications interfere | | 24-hour urine metanephrines | 88–96% | 85–89% | Good sensitivity; less affected by stress than catecholamines | Requires 24-hour collection; patient compliance issues | | Plasma catecholamines | 80–90% | 70–80% | Readily available | Less specific; affected by stress, posture, medications | | 24-hour urine catecholamines | 60–80% | 70–80% | Older standard | Poor sensitivity; affected by many variables | **Warning:** A single normal plasma free metanephrines level has >95% negative predictive value and effectively excludes pheochromocytoma. Repeat testing is rarely needed unless clinical suspicion remains very high. ## Role of Other Investigations ### 24-Hour Urine Metanephrines While acceptable, this is less convenient than plasma free metanephrines and is now considered a second-line confirmatory test. It may be used if plasma testing is equivocal or unavailable. ### CT/MRI Abdomen and Pelvis This is a **localization** study, not a confirmatory test. It should only be performed AFTER biochemical confirmation. Imaging without biochemical confirmation risks false-positive findings (incidentalomas). ### MIBG Scintigraphy This is a **functional imaging** study used for: - Detecting extra-adrenal pheochromocytomas (paragangliomas) - Identifying metastatic disease - Assessing for multiple tumors (in familial syndromes) It is performed AFTER biochemical confirmation and anatomical localization with CT/MRI. **Mnemonic:** **CONFIRM then LOCATE then FUNCTION** — First confirm biochemically (plasma metanephrines), then locate anatomically (CT/MRI), then assess function/metastases (MIBG).