Total Free (MN+NMN)

Some of the most severe blood pressure spikes, pounding headaches, and panic-like episodes come from a small, hormone-producing tumor that standard blood work misses entirely. This test picks up the chemical fingerprint those tumors leave in your blood with sensitivity approaching 100 percent in most published series.

If you have unexplained blood pressure swings, an adrenal mass found on imaging, or a family history of certain endocrine tumors, this is the first biochemical test endocrinologists reach for. It can also reveal more subtle, ongoing increases in the body's stress hormone system that have been linked to higher cardiovascular and kidney risk.

What This Test Actually Measures

The test measures two molecules together in your plasma: MN (metanephrine) and NMN (normetanephrine). These are the breakdown products your body creates when it processes adrenaline and noradrenaline, the hormones your adrenal glands release during stress. Adding them together gives a single number that reflects how much of these hormones your body is producing day and night.

The 'free' part matters. These molecules travel in your blood in two forms, an unbound (free) form and a chemically tagged (conjugated) form. Free metanephrines clear quickly from the circulation, so they give an almost real-time readout of ongoing hormone production rather than a long-term average. This is why they have become the preferred marker over older catecholamine or VMA (vanillylmandelic acid) tests.

Why Your Level Matters

The hormones your body breaks down into MN and NMN come mostly from specialized cells in the center of your adrenal glands, with smaller contributions from clusters of similar cells along the spine and neck. When one of these cell clusters grows into a tumor called PPGL (pheochromocytoma or paraganglioma), it can pump out adrenaline-type hormones in huge amounts. MN and NMN rise in step, often to many times the normal upper limit.

The pattern of which marker rises tells you something about where the tumor lives. Adrenal tumors that produce adrenaline tend to push MN higher. Tumors elsewhere in the body that produce mostly noradrenaline tend to push NMN higher. Research on 291 people with confirmed PPGL showed that plasma free metanephrines could accurately predict tumor size and location based on these patterns alone.

Heart and Kidney Risk Beyond Tumors

Even without a tumor, persistently higher metanephrine levels have been tied to real cardiovascular consequences. Studies measuring urinary metanephrines (a related but different measurement) in 1,374 adults found a strong association between higher levels and hypertensive heart disease, metabolic syndrome, and early kidney damage. A second study of 828 people with non-functioning adrenal masses found similar links, with the authors recommending that people in the upper ranges be treated more aggressively for cardiometabolic risk.

Whether plasma free MN and NMN follow the exact same pattern has not been as extensively studied as urine. The biology is the same, the markers come from the same pathway, but evidence tying plasma values directly to long-term heart and kidney outcomes is still accumulating. A modestly elevated plasma result that does not cross the tumor threshold is a signal worth investigating for broader sympathetic nervous system activation, not a result to dismiss.

How Well the Test Actually Performs

A 2017 meta-analysis of plasma free metanephrines for PPGL detection found sensitivity of roughly 97 percent and specificity of 94 percent, with an AUC (a statistical measure of overall accuracy where 1.0 is perfect) of 0.99. A prospective study of 2,056 patients across risk levels found the combined MN plus NMN panel caught between 96.6 and 97.9 out of every 100 tumors, with specificity between 94 and 95 percent. A large low-risk cohort of 1,260 people with hypertension showed sensitivity of 100 percent and specificity of 96.7 percent, meaning no tumors were missed.

Compared with older tests, this is a meaningful upgrade. Urinary catecholamines and VMA typically catch only 72 to 84 out of 100 cases. Plasma catecholamines also underperform. This is why the Endocrine Society's clinical practice guideline names plasma free or urinary fractionated metanephrines as the preferred first-line biochemical test for PPGL.

Reference Ranges

Reference ranges for plasma free metanephrines depend heavily on the assay, your age, and whether the blood was drawn while you were seated or lying down. Values from one method do not translate directly to another. The numbers below come from an LC-MS/MS assay validation study and a large reference population, and are offered for orientation rather than as universal targets. Your lab will report its own cutpoints, and you should compare future results against your own baseline within the same lab.

Source: Cutpoints drawn from Eisenhofer 2013 and Grouzmann 2010. The 2x upper limit threshold has been shown to sharply improve specificity without meaningfully reducing sensitivity in adult populations.

When Results Can Be Misleading

A single reading can mislead if you do not control for the factors that genuinely shift these molecules in your blood. These shifts are not disease, they are physiology.

• Kidney function: plasma free MN and NMN are frequently elevated in chronic kidney disease and people on dialysis, sometimes overlapping with tumor ranges. Specialized reference intervals exist for people with reduced kidney function.
• Body position during the draw: sitting upright produces higher values than lying down, especially for NMN. A result above the cutoff drawn seated may be normal when drawn after 30 minutes of rest in a semi-recumbent position.
• Acute exercise: running and other intense activity raise free MN and NMN in proportion to effort. Values normalize within hours. Avoid heavy exercise for 24 hours before the test.
• Diet: a diet heavy in catecholamine-rich foods has a negligible effect on free MN and only minimal effect on free NMN, which is one reason free (rather than total) metanephrines are preferred.

Drug confounders are also worth knowing. Tricyclic antidepressants, certain antipsychotics, and alpha-adrenergic blockers can shift readings without causing PPGL. If you are on any of these and your result is elevated, the result should be interpreted cautiously rather than treated as confirmation of a tumor.

Tracking Your Trend and Acting on the Result

This is not a biomarker where one reading tells the whole story. A borderline elevation on a single draw is common when pre-test conditions are not controlled, and it does not mean a tumor is present. A reading well above twice the upper limit, especially if you have symptoms or an adrenal mass, is a different situation that calls for prompt follow-up.

For most healthy adults, a baseline plus a retest if the first is borderline is usually enough. If you have hereditary risk (MEN2, von Hippel-Lindau, NF1, or SDHx mutations) or prior PPGL, repeat testing at least annually, and sooner if symptoms appear. For children with these hereditary risks, using pediatric-specific reference ranges has been shown to detect tumors roughly a year earlier than relying on adult cutoffs.

If your result is high, the next steps are structured. Repeat the draw under optimized conditions: early morning, fasting, after 30 minutes of rest lying down, and off interfering medications if your doctor agrees it is safe. If the repeat result is still elevated, adrenal and whole-body imaging (CT or MRI) is the next step, alongside referral to an endocrinologist. For strongly elevated results, adding plasma methoxytyramine testing can help detect tumors outside the adrenal gland and inform genetic workup. Chromogranin A is another complementary test sometimes ordered to support the picture.