Metanephrine- Free Plasma

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Report in 96Hrs

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Fasting Required

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Catecholamine metabolites.

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Metanephrine- Free Plasma Test Information Guide

Why is it done?
- Detects the presence of metanephrine, a metabolite of the catecholamine epinephrine produced by the adrenal medulla and sympathetic nervous system
- Primary indication is screening for pheochromocytoma, a rare neuroendocrine tumor that secretes excessive catecholamines causing severe hypertension
- Recommended for patients presenting with symptoms of catecholamine excess including severe headaches, profuse sweating, palpitations, and sudden blood pressure elevations
- Used to evaluate unexplained hypertension, particularly in patients with resistant hypertension or hypertension resistant to multiple antihypertensive medications
- Performed in patients with family history of pheochromocytoma or associated genetic syndromes such as MEN2A, MEN2B, Von Hippel-Lindau disease, and Neurofibromatosis Type 1
- Used for monitoring patients with diagnosed pheochromocytoma before and after surgical treatment or during medical management
- Typically ordered during initial diagnostic workup when pheochromocytoma is clinically suspected based on presentation and vital signs
Normal Range
- Normal reference range: Less than 90 pg/mL (or <0.5 nmol/L) for plasma free metanephrine Normal reference range: Less than 113 pg/mL (or <0.6 nmol/L) for plasma free normetanephrine
- Reference ranges may vary slightly between laboratories depending on analytical methods, patient positioning (supine vs upright), and collection techniques
- Negative result: Metanephrine levels within normal range suggest pheochromocytoma is unlikely and generally rules out the diagnosis with high confidence
- Positive result: Metanephrine levels exceeding the upper limit of normal strongly suggest pheochromocytoma and warrant further diagnostic investigation
- Borderline or mildly elevated values (90-200 pg/mL) may require repeat testing and clinical correlation as some false positives can occur with stress, medications, or improper sample collection
- Markedly elevated values (>400 pg/mL) have very high specificity for pheochromocytoma and virtually exclude normal physiology
- Units of measurement: pg/mL (picograms per milliliter) or nmol/L (nanomoles per liter)
Interpretation
- Elevated metanephrine with normal normetanephrine: May indicate tumor secreting primarily epinephrine, typically originating from the adrenal medulla
- Elevated normetanephrine with normal metanephrine: May suggest extraadrenal pheochromocytoma (paraganglioma) or tumors secreting primarily norepinephrine
- Both metanephrine and normetanephrine elevated: Classic pattern highly suggestive of pheochromocytoma with mixed catecholamine secretion
- Normal results: Effectively exclude pheochromocytoma with >95% sensitivity when patient is in supine position and properly prepared; has excellent negative predictive value
- Factors affecting results include medications (decongestants, certain antidepressants, sympathomimetics), caffeine consumption, acute stress, pain, physical activity, and smoking immediately before testing
- Cold exposure, emotional stress, and strenuous exercise can transiently elevate catecholamine metabolites; repeat testing under controlled conditions may be needed
- Patient positioning is critical: supine position for at least 5-30 minutes before sampling gives most reliable results; standing position yields falsely elevated values
- Plasma metanephrine testing has superior diagnostic performance compared to urine metanephrine testing with higher sensitivity and specificity
- Test is most useful as initial screening tool; elevated results typically lead to imaging studies (CT or MRI of abdomen/pelvis) or functional imaging (MIBG scan, PET scan) for tumor localization
Associated Organs
- Primary organ: Adrenal medulla (innermost portion of adrenal glands) which produces epinephrine and norepinephrine; sympathetic nervous system ganglia can also secrete catecholamines
- Pheochromocytoma: A catecholamine-secreting neuroendocrine tumor arising from chromaffin cells; 80-85% originate in adrenal medulla, 15-20% are extraadrenal (paragangliomas)
- Target organs of excessive catecholamines include the cardiovascular system (heart, blood vessels), nervous system, metabolic organs (pancreas), and kidneys
- Associated complications from catecholamine excess include severe hypertension, myocardial infarction, stroke, cardiomyopathy, acute coronary syndrome, and arrhythmias
- Genetic syndromes associated with pheochromocytoma include: MEN2A (multiple endocrine neoplasia), MEN2B, Von Hippel-Lindau disease, Neurofibromatosis Type 1 (NF1), and familial paraganglioma syndromes
- Untreated pheochromocytoma can cause life-threatening complications including hypertensive crisis, acute myocardial infarction, acute cerebrovascular accident, acute pulmonary edema, and sudden cardiac death
- Related conditions: Malignancy-associated hypertension, cocaine/amphetamine use (causing false elevation), hyperthyroidism, anxiety disorders, and other causes of sympathomimetic excess
Follow-up Tests
- 24-hour urine metanephrine and catecholamine testing: Alternative confirmatory test; if plasma metanephrine elevated, urine testing may provide additional confirmation
- Plasma chromogranin A level: Additional diagnostic marker for neuroendocrine tumors; elevated in ~80% of pheochromocytomas; higher specificity when both metanephrine and chromogranin A are elevated
- Abdominal and pelvic CT scan with contrast: First-line imaging study for tumor localization when plasma metanephrine is elevated; can identify most adrenal pheochromocytomas
- Abdominal MRI: Alternative imaging modality when CT is contraindicated or for better characterization of indeterminate adrenal masses; superior for detecting extraadrenal paragangliomas
- I-123 or I-131 MIBG (metaiodobenzylguanidine) scintigraphy: Functional imaging for confirming catecholamine-secreting tumor; particularly useful for detecting metastases or extraadrenal tumors
- PET scan with Fluorodopa or other radiotracers: Advanced imaging for improved localization and staging of pheochromocytoma; useful for finding metastatic disease
- Genetic testing: Recommended for all patients with confirmed pheochromocytoma to screen for hereditary syndromes (RET, NF1, VHL, SDHA/B/C/D mutations)
- Blood pressure monitoring and orthostatic vital signs: Serial measurements to assess blood pressure trends and response to therapy
- Repeat plasma metanephrine testing: Recommended 6-12 months post-treatment to confirm surgical success; baseline test before surgery helps with post-operative follow-up comparison
- Annual surveillance: Recommended even after successful treatment as recurrence can occur; malignant pheochromocytomas have recurrence rates up to 50%
- ECG and cardiac evaluation: Assessment for catecholamine-induced cardiac complications, particularly with markedly elevated catecholamine levels
Fasting Required?
- Fasting: No - Food intake does not need to be restricted prior to this test
- Adequate rest and relaxation: Patient should rest in supine position (lying flat) for 5-30 minutes before blood draw; this standardized positioning is essential for accurate results
- Avoid caffeine: Abstain from caffeine-containing beverages and foods (coffee, tea, chocolate, energy drinks) for at least 24-48 hours before testing as caffeine stimulates catecholamine release
- Avoid decongestants: Discontinue nasal decongestants (pseudoephedrine, phenylephrine), appetite suppressants, and stimulant medications for at least 48 hours before test; these medications can falsely elevate results
- Avoid certain medications: Discuss with healthcare provider about temporarily discontinuing tricyclic antidepressants, decongestants, sympathomimetics, and stimulating medications; inform laboratory of all current medications
- Avoid smoking: Do not smoke for at least 24 hours before testing; nicotine can elevate catecholamine levels
- Minimize stress and physical activity: Avoid strenuous exercise, emotional stress, and physical exertion for 24 hours before testing; remain calm during sample collection
- Avoid cold exposure: Do not expose body to extreme cold before testing; cold exposure triggers catecholamine release
- Appropriate timing: Testing is ideally performed in the morning between 7 AM and 11 AM when catecholamine fluctuations are minimal
- Avoid alcohol: Discontinue alcohol consumption for at least 24 hours before testing; alcohol can affect blood pressure and catecholamine levels
- Verify medication list: Provide complete list of all over-the-counter and prescription medications to healthcare provider before testing; confirm no contraindicated substances are being taken

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