metanephrine

Metanephrine is a metabolite of epinephrine formed when catechol-O-methyltransferase (COMT) methylates epinephrine. It is one of the catecholamine metabolites produced in the adrenal medulla, peripheral tissues, and chromaffin-related tumors. Metanephrine, like its counterpart normetanephrine, is more chemically stable than the parent catecholamines, which underlies its use in diagnostic testing.

Clinically, metanephrine measurements are used to screen for pheochromocytoma and paraganglioma. Plasma free metanephrine and normetanephrine, or urinary fractionated metanephrines, are commonly assessed. Elevated levels of these metabolites indicate excessive catecholamine production and support a diagnosis of a catecholamine-secreting tumor; normal results make such tumors less likely, though not impossible.

Biochemical pathway and excretion: Epinephrine is converted to metanephrine, and norepinephrine to normetanephrine, via COMT. These metabolites are subsequently eliminated from the body, with urinary excretion of metanephrine and normetanephrine serving as a standard diagnostic measure. Vanillylmandelic acid (VMA) is a downstream metabolite in the same pathway.

Interpretation and limitations: Test results can be influenced by medications (for example, certain antidepressants and beta-blockers), acute stress, and sampling conditions. Proper patient preparation and validated assay techniques improve accuracy, and positive results are typically followed by imaging studies to locate tumors.