Pheo Biochemical Confirmation

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name: pheo-biochemical-confirmation
description: Algorithm for interpreting metanephrine results and handling borderline elevations in suspected Pheochromocytoma. Trigger when a clinician asks "how to interpret metanephrines", "metanephrine units", or "borderline metanephrine result".
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# Pheochromocytoma Biochemical Confirmation

This skill guides the interpretation of biochemical tests for Pheochromocytoma and Paraganglioma (PPGL) according to Endocrine Society guidelines.

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## Step 1 — Select the Correct Initial Test

| Patient Risk Level | Preferred Test |
|---|---|
| **High Risk** (Genetic syndrome, Adrenal mass, Triad) | **Plasma free metanephrines** (High sensitivity) |
| **Low Risk** (Refractory HTN, unusual BP swings) | **24-hour urinary fractionated metanephrines** (High specificity) |

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## Step 2 — Interpret the Results

Compare results to the upper limit of normal (ULN):

1. **> 3-4x ULN**: Diagnostic for Pheo/PPGL. Move to **Imaging**.
2. **Normal (< 1x ULN)**: Pheo/PPGL ruled out (unless patient was symptomatic during a normal 24h urine collection).
3. **Borderline (1x to 3x ULN)**: High risk for false positives. Proceed to Step 3.

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## Step 3 — Resolve Borderline Elevations

1. **Review Medications**: Stop TCAs, SNRIs, Levodopa, and Decongestants for at least 2 weeks, then repeat the test.
2. **Suppression Testing**: Consider a **Clonidine Suppression Test** (0.3 mg dose):
   - Measure plasma normetanephrine at 0 and 3 hours.
   - Failure to suppress plasma normetanephrine by >40% or below ULN confirms Pheo.

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## Clinical Guardrails

- **False Positives**: Common causes include critical illness, OSA, caffeine, and improper sampling (Plasma should be drawn after 20 mins of supine rest).
- **Imaging Lock**: Do NOT order CT or MRI until the biochemical result is at least 3x ULN or confirmed by suppression testing.

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**Source:** Lenders JW, et al. Pheochromocytoma and Paraganglioma: An Endocrine Society Clinical Practice Guideline. *J Clin Endocrinol Metab*. 2014.