Plasma Metanephrines Test

A plasma metanephrines test measures breakdown products of adrenaline and noradrenaline, the body's "fight-or-flight" hormones. It is used mainly to screen for rare tumours of the adrenal glands and related nerve tissue that can release these hormones in excess.

What the test measures

The adrenal glands and certain nerve tissue make catecholamines, including adrenaline (epinephrine) and noradrenaline (norepinephrine). As the body clears these hormones, it converts them into metanephrine and normetanephrine. Because these breakdown products are produced steadily, even when hormone release comes in bursts, measuring them can be a sensitive way to detect overproduction that intermittent hormone surges might otherwise hide.

The test usually reports plasma free metanephrines, sometimes alongside a urine version of the same measurement. Elevated levels suggest the body is processing an excess of catecholamines, which raises the question of a hormone-secreting tumour such as a pheochromocytoma or paraganglioma.

Why breakdown products are measured instead of the hormones themselves

Adrenaline and noradrenaline can be released in short, unpredictable surges, so a single blood level of the hormones can easily miss the overproduction. Metanephrine and normetanephrine, by contrast, are formed continuously as the hormones are processed. Measuring these steadier products gives a more reliable signal of whether catecholamine production is sustained and excessive, which is the reason this test is favored as a screen.

Why a clinician might order it

A clinician may order metanephrines when symptoms suggest catecholamine excess, such as episodes of severe or hard-to-control high blood pressure, a pounding heartbeat, sweating spells, and headaches occurring together. It is also ordered when an adrenal mass is found incidentally on imaging, or when there is a personal or family history of conditions associated with these tumours. The test serves as a primary screening tool for pheochromocytoma and paraganglioma.

How the test generally works

Plasma metanephrines are measured from a blood sample drawn from a vein. Because posture and activity affect the result, many laboratories ask you to rest quietly before the draw, sometimes lying down for a set period, so the sample reflects a calm baseline. A urine collection version of the test may be used instead of or in addition to the blood test. The assays are specialized, and the laboratory method influences the reported value, so results are read against that laboratory's own reference range.

How to prepare

Preparation is important because many everyday factors can raise metanephrines and cause false positives. Laboratories often ask you to rest quietly, sometimes lying down, for a period before the blood draw. Caffeine, certain foods, stress, strenuous activity, and several medications can affect results. Share a complete medication list, including over-the-counter products, and follow the specific preparation instructions you are given.

What can affect the result

• Posture and activity: standing, exertion, or testing without a rest period can raise values.
• Caffeine and certain foods: some dietary factors can elevate the result.
• Stress and acute illness: physical or emotional stress increases catecholamine turnover.
• Medications: several drugs, including some that affect the nervous system, can raise metanephrines.
• Collection conditions: a draw taken without the recommended rest is more likely to read high.

Plasma versus urine metanephrines

Metanephrines can be measured in blood or in a 24-hour urine collection, and each approach has its place. The plasma test is convenient and can be drawn at a single visit, although it is sensitive to posture and activity, which is why a rest period before the draw is often requested. The urine collection gathers the body's output across a full day, which can smooth out brief fluctuations but depends on collecting every urination completely. A clinician chooses between them, or uses both, based on the clinical question and the practicalities of testing, and reads either result against the laboratory's own reference range.

Whichever sample is used, the principle is the same: the test looks for sustained overproduction of catecholamines by measuring their steady breakdown products. The strength of the test as a screen comes from its tendency to be normal when no overproducing tumour is present, which helps reassure when results are clearly within range.

How results are generally interpreted

Metanephrines are interpreted against the degree of elevation and the clinical context, by a clinician rather than from a number alone.

• Markedly elevated metanephrines raise concern for a catecholamine-secreting tumour and typically prompt confirmatory testing and imaging.
• Mildly elevated metanephrines are common and often reflect medications, stress, caffeine, or test conditions rather than a tumour, so they are usually rechecked under controlled conditions.
• Normal metanephrines make a catecholamine-secreting tumour unlikely, which is why the test is valued as a screen.

Illustrative reference ranges

The values below are illustrative only and vary by laboratory, assay, posture, age, and sex. Always use the range printed on your own report and interpret with attention to test conditions.

Related tests

Metanephrines are part of evaluating the adrenal glands and difficult-to-control blood pressure, and may be ordered with the renin test when an adrenal cause of hypertension is being explored. See the blood tests index for related adrenal tests, conditions for background on adrenal disorders, and the hormones index for catecholamine physiology. The guides index can help with reading the report.