The plasma free metanephrines is used to diagnose the presence of a rare tumor known as pheochromocytoma that releases out excess metaphrines. Testing can also be used when the tumor has been removed and treated to monitor its recurrence. Epinephrine and norpinephrine are hormones called catecholamine that is released in the blood in response to physical and mental stress and help in regulating blood pressure throughout the body. Metanephrines and normetanephrines are breakdown products of these hormones. Urine metanephrines test or plasma free metanephrines test evaluates an individual for pheochromocytoma. The blood test in most useful when a person has persistent hypertension. The hormones do not stay in the blood for long. Urine test checks the presence of hormones for 24 hours.