Plasma metanephrines

Metanephrines are the stable breakdown products of the 'fight-or-flight' hormones adrenaline (epinephrine) and noradrenaline (norepinephrine), together called catecholamines. Because a tumour that makes catecholamines produces these metabolites continuously — even between symptom spells — measuring the metanephrines in the blood is one of the most sensitive ways to catch it. The main use is to screen for and diagnose a pheochromocytoma, a rare tumour of the adrenal gland (or a closely related tumour called a paraganglioma), which floods the body with catecholamines. The classic picture is episodes of severe high blood pressure with headache, heavy sweating, a pounding heart and pallor, though many cases are subtler or found while investigating hard-to-control hypertension or an incidentally discovered adrenal lump. The test's great strength is that a normal result makes the diagnosis very unlikely, so it is trusted to rule the tumour out. A clearly raised level is a strong signal that needs confirming and localising with imaging. Because certain medicines, caffeine, stress and even the position you are lying in can nudge the result, the test is done under specific conditions and interpreted carefully.

• Pheochromocytoma or paraganglioma — the key concern; a catecholamine-secreting tumour.
• A clearly high level (several times the upper limit) — strongly suggests a tumour and prompts imaging to locate it.
• A mild elevation — is often a false positive from medications (some antidepressants, decongestants), caffeine, acute stress or illness; usually repeated under proper conditions.
• Severe physical stress — major illness or exertion can raise catecholamines.

• Normal (in range) — the reassuring and expected result; makes a pheochromocytoma very unlikely.