A 38-year-old woman from rural Maharashtra presents to the emergency department with severe headache, profuse sweating, and palpitations for the past 2 hours. Her blood pressure is 210/130 mmHg with a heart rate of 118 bpm. She reports episodic attacks of similar symptoms over the past 6 months, each lasting 15–45 minutes and followed by profound fatigue. On examination, she appears anxious and diaphoretic. Fundoscopy reveals grade II hypertensive retinopathy. Her serum glucose is 156 mg/dL, and ECG shows sinus tachycardia with T-wave inversions in the precordial leads. 24-hour urine metanephrines are elevated at 1.8 times the upper limit of normal. What is the most appropriate next diagnostic step?

Explanation

## Diagnostic Approach to Pheochromocytoma ### Clinical Presentation Recognition **Key Point:** The classic triad of episodic headache, profuse sweating, and palpitations with severe hypertension in a young patient is pathognomonic for pheochromocytoma. The elevated 24-hour urine metanephrines already suggest biochemical confirmation of catecholamine excess. ### Biochemical Confirmation Algorithm **High-Yield:** Plasma free metanephrines (measured supine after 30 minutes of rest in a quiet room) is the gold standard initial confirmatory test after a positive screening test (elevated 24-hour urine metanephrines). It has superior sensitivity (96%) and specificity (85%) compared to other biochemical markers. **Mnemonic: PLASMA-FIRST** — **P**lasma free metanephrines are **L**ead test, **A**fter **S**creening positive, **M**easured **A** supine, **F**or **I**nitial **R**ole in confirming, **S**uperior **T**o others. ### Why Plasma Free Metanephrines? | Test | Sensitivity | Specificity | Timing | Advantage | |------|-------------|-------------|--------|----------| | Plasma free metanephrines | 96% | 85% | Supine, 30 min rest | Gold standard confirmatory test | | 24-hour urine metanephrines | 88% | 88% | Already done (positive) | Screening test | | Urine VMA | 60% | 90% | Less sensitive | Older method, lower sensitivity | | Clonidine suppression | Variable | 80% | Suppresses normal catecholamines | Used only if plasma metanephrines borderline | **Clinical Pearl:** Plasma metanephrines must be drawn in the supine position after 30 minutes of rest in a quiet, temperature-controlled room to minimize false positives from stress and postural changes. The patient should avoid caffeine, decongestants, and tricyclic antidepressants for 1 week prior. ### Localization vs. Confirmation **Key Point:** Biochemical confirmation (plasma free metanephrines) must precede anatomical localization (CT/MRI). Localizing a tumor in a biochemically unconfirmed patient risks unnecessary intervention and misdiagnosis. ### Sequential Diagnostic Pathway ```mermaid flowchart TD A[Clinical suspicion of pheochromocytoma]:::outcome --> B[24-hour urine metanephrines]:::action B --> C{Elevated?}:::decision C -->|Yes| D[Plasma free metanephrines<br/>supine, 30 min rest]:::action D --> E{Elevated?}:::decision E -->|Yes| F[Biochemically confirmed]:::outcome F --> G[CT/MRI abdomen + pelvis<br/>for localization]:::action E -->|Borderline| H[Clonidine suppression test]:::action C -->|No| I[Pheochromocytoma unlikely]:::outcome G --> J[Genetic testing if:<br/>Age <40, bilateral, familial,<br/>malignant features]:::action ``` [cite:Harrison 21e Ch 405]