Fractionated Free Metanephrines

Your adrenal glands sit on top of your kidneys and produce hormones that control your heart rate, blood pressure, and stress response. Sometimes a tumor grows inside one of these glands (or in nearby nerve tissue) and floods your body with adrenaline and related hormones. These tumors are called pheochromocytomas (when they arise in the adrenal gland) or paragangliomas (when they arise outside it). They are rare, but when present they can cause life-threatening blood pressure crises, strokes, and heart attacks.

This panel measures the steady breakdown products of those adrenaline hormones circulating in your blood, rather than the hormones themselves. That distinction matters. Adrenaline (epinephrine) and noradrenaline (norepinephrine) surge and crash throughout the day in response to stress, exercise, and posture changes. Their breakdown products, called free metanephrines, are produced continuously by tumor tissue and remain far more stable in the bloodstream. A study of 858 patients found that plasma free metanephrines detected 99% of pheochromocytomas, making this the single most sensitive screening test available for these tumors.

What This Panel Reveals

The panel separates the signal into three measurements, each telling a different part of the story. Free metanephrine comes from the breakdown of epinephrine, the hormone that drives your fight-or-flight response. Free normetanephrine comes from norepinephrine, the hormone that maintains baseline blood vessel tone and blood pressure. The total combines both values into a single number.

This separation is not just academic. Different tumor types produce different hormone profiles. Pheochromocytomas that arise inside the adrenal gland typically elevate both metanephrine and normetanephrine, because the adrenal medulla (the hormone-producing core of the adrenal gland) is the main site where norepinephrine gets converted into epinephrine. Paragangliomas that grow outside the adrenal gland usually elevate only normetanephrine, because those tissues lack the enzyme needed to make epinephrine.

Recognizing which fraction is elevated helps your physician determine where a tumor is likely located and whether it may be linked to an inherited genetic syndrome. Hereditary forms of these tumors, which account for roughly 30% to 40% of cases according to large registry studies, often show distinct biochemical patterns. Tumors linked to mutations in the VHL (von Hippel-Lindau) gene or SDH (succinate dehydrogenase) genes tend to produce normetanephrine predominantly, while those linked to MEN2 (Multiple Endocrine Neoplasia type 2) elevate metanephrine as well.

How to Read Your Results Together

Interpretation depends on how far above the upper reference limit your values fall. The Endocrine Society clinical practice guideline recommends a straightforward threshold approach: values more than three times the upper limit of normal carry a very high probability of a true tumor and should prompt immediate imaging. Values that are mildly elevated (between one and three times the upper limit) sit in a gray zone where false positives are common and further testing is needed.

The total free metanephrines value (the sum of metanephrine and normetanephrine) provides a quick screening number, but it is the individual fractions that guide clinical decisions. A normal total with a mildly elevated normetanephrine could still be meaningful. Always look at each fraction individually.

When Results Can Be Misleading

False positives are the main challenge with this panel. Several common medications raise plasma metanephrines and can mimic the biochemical signature of a tumor. Tricyclic antidepressants, certain ADHD stimulants, monoamine oxidase inhibitors, and some decongestants containing adrenaline-like compounds can all elevate results. Acetaminophen (Tylenol) can interfere with certain laboratory methods and produce falsely high readings unrelated to actual hormone levels.

Body position during the blood draw also matters. Standing or sitting upright activates your fight-or-flight nervous system (the sympathetic nervous system), which raises norepinephrine and its breakdown products. Studies have shown that drawing blood after 20 to 30 minutes of lying flat (supine sampling) reduces false positive rates significantly. If your results come back mildly elevated from a seated draw, a repeat test in the supine position can clarify whether the elevation is real.

Acute physical or emotional stress, including anxiety about the blood draw itself, can temporarily raise these hormone breakdown products. Kidney impairment also increases plasma metanephrine levels because the kidneys help clear these compounds from the blood. If your kidney function markers (creatinine or cystatin C) are elevated, your physician should factor that into the interpretation.

Who Should Consider This Panel

The Endocrine Society guideline recommends biochemical testing for pheochromocytoma and paraganglioma in specific clinical scenarios: high blood pressure that resists treatment, or sudden episodic blood pressure spikes; an adrenal mass discovered incidentally on imaging; a family history of pheochromocytoma, paraganglioma, or related genetic syndromes such as MEN2, von Hippel-Lindau disease, neurofibromatosis type 1, or SDH gene mutations; and symptoms of excess adrenaline, such as episodic headaches, sweating, and rapid heartbeat occurring together.

About 5% to 7% of adrenal masses found incidentally on CT or MRI scans turn out to be pheochromocytomas. If you have been told you have an adrenal incidentaloma, this panel should be part of the initial workup. For anyone with a known genetic predisposition (carrier of an SDH, VHL, RET, or NF1 mutation), periodic screening with plasma free metanephrines is recommended even in the absence of symptoms, because tumors can grow silently for years before producing noticeable episodes.

Tracking Over Time

For most people, a single normal result is reassuring and no routine repeat is needed. The value of serial testing comes in three specific situations. First, if you carry a genetic mutation that predisposes you to these tumors, annual screening with this panel is standard practice. These hereditary tumors can appear at any age, and catching them before they cause a dangerous blood pressure emergency is the entire point of surveillance.

Second, after surgical removal of a pheochromocytoma or paraganglioma, serial testing detects recurrence. Up to 17% of tumors that appear benign at surgery recur or develop metastatic disease later, according to long-term follow-up studies. Post-surgical monitoring with this panel, typically at 6 to 12 month intervals for at least 10 years, is the standard of care.

Third, if your initial result was mildly elevated and follow-up testing was inconclusive, tracking the trend over two or three measurements several weeks apart helps separate a slowly growing tumor from a stable false positive. A true tumor tends to show progressive increases over time, while false positives from medications or stress remain flat or fluctuate randomly.

What to Do with Your Results

If all three values are within normal limits, the probability that you have a pheochromocytoma or paraganglioma is extremely low (less than 1%). No additional workup for excess adrenaline-type hormones is needed. If one or both fractions are elevated above three times the upper reference limit, the next step is imaging, typically a CT or MRI of the abdomen and pelvis. An endocrinologist should guide this workup.

For mildly elevated results, do not panic. Ask your physician whether the blood was drawn under ideal conditions (supine, fasting, no interfering medications). A repeat test under controlled conditions resolves most borderline cases. If the repeat is still elevated, a clonidine suppression test can help distinguish a true tumor from an overactive fight-or-flight response. Clonidine normally suppresses norepinephrine release; if metanephrines fail to drop after clonidine, a tumor source becomes more likely.

If a tumor is confirmed, genetic counseling and genetic testing should be considered. The high rate of hereditary disease (30% to 40% of all cases) means that identifying a genetic cause has implications for your family members, who may benefit from their own screening. The specialist managing your case will typically be an endocrinologist working with a surgeon experienced in adrenal surgery.