The Test Sample
What is being tested?
The plasma free metanephrines test measures the amount of metanephrine and normetanephrine in the blood. These substances are metabolites of epinephrine (adrenaline) and norepinephrine. Epinephrine and norepinephrine are catecholamine hormones that help regulate the flow and pressure of blood throughout the body and play important roles in the body's response to stress.
Catecholamines are produced in the medulla – the interior portion of the adrenal glands – and secreted into the blood. Once these hormones have completed their actions, they are metabolized to inactive compounds. Norepinephrine breaks down into normetanephrine and vanillylmandelic acid (VMA) and epinephrine becomes metanephrine and VMA. Both of the catecholamines and their metabolites are normally found in small fluctuating quantities in the blood and urine.
A rare tumor called a pheochromocytoma can produce large amounts of catecholamines, resulting in significantly increased concentrations of metanephrine and normetanephrine. About 90% of pheochromocytomas form in the adrenal glands and, while a few are cancerous, most are benign – they continue to grow but usually do not spread beyond their original location.
The catecholamines produced by pheochromocytomas can cause persistent hypertension and episodes of severe high blood pressure. This can cause symptoms such as headaches, palpitations, sweating, nausea, anxiety, and tingling in the extremities. Left untreated, the symptoms may worsen as the pheochromocytoma grows. Over time, hypertension caused by the tumor may cause kidney damage, heart disease, and raise the risk of a stroke or heart attack.
Several tests, including plasma free metanephrines, can detect the presence of pheochromocytomas. Although they are rare, it is important to diagnose these tumors because they cause a potentially curable form of hypertension. In most cases, the pheochromocytomas can be surgically removed, which eliminates the high blood pressure and its associated symptoms and complications.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. Although there is some disagreement over the specifics of how the sample should be collected, it may be necessary to lie down and rest quietly for 15 to 30 minutes prior to and during sample collection. In other circumstances, you may just be seated upright with little rest time before the sample collection.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Preparation for the test is important for accurate results. It may be necessary to discontinue epinephrine and epinephrine-like drugs for at least one week before the test, stop using acetaminophen 48 hours beforehand, and fast for 8-10 hours prior to collection. It is especially important not to have any caffeine-containing food (soda, chocolate), coffee (including decaf), tobacco (smoking cigarettes or cigars), tea, or alcohol for at least 4 hours before specimen collection. Talk to the doctor about all medications being taken. Since several different drugs may interfere with the test, he may instruct the person being tested to stop taking all medications except those that are necessary for one week prior to the test. However, do not stop taking any medications without first consulting the doctor.