| 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". |
Pheochromocytoma Biochemical Confirmation
This skill guides the interpretation of biochemical tests for Pheochromocytoma and Paraganglioma (PPGL) according to Endocrine Society guidelines.
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) |
Step 2 — Interpret the Results
Compare results to the upper limit of normal (ULN):
- > 3-4x ULN: Diagnostic for Pheo/PPGL. Move to Imaging.
- Normal (< 1x ULN): Pheo/PPGL ruled out (unless patient was symptomatic during a normal 24h urine collection).
- Borderline (1x to 3x ULN): High risk for false positives. Proceed to Step 3.
Step 3 — Resolve Borderline Elevations
- Review Medications: Stop TCAs, SNRIs, Levodopa, and Decongestants for at least 2 weeks, then repeat the test.
- 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.
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.
Source: Lenders JW, et al. Pheochromocytoma and Paraganglioma: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2014.
