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 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. They are produced in the medulla – the interior portion of the adrenal glands. The catecholamines that the adrenal glands produce, and their metabolites, metanephrine and normetanephrine, are normally found in small fluctuating quantities in both 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 in both the blood and urine. 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/or paroxysms (bouts) 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 damage organs such as the kidneys and heart and raise the risk of an affected patient having 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 pheochromocytomas because they cause a potentially curable form of hypertension. In most cases, the tumors 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 your arm. Although there is some disagreement over the specifics of how the sample should be collected, you may be asked to lie down and rest quietly for 15 – 30 minutes prior to sample collection, and your blood may be collected while you are lying down. In other circumstances, you may just be seated upright with little or no rest time before the sample collection.

Is any test preparation needed to ensure the quality of the sample?
Preparation for the test is important for accurate results. You should discontinue epinephrine and epinephrine-like drugs for at least 1 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 your doctor about all the medications you are taking.  Since several different drugs may interfere with the test, he may instruct you to withhold all medications except those that are necessary for one week prior to the test.  However, do not stop taking any medications without first consulting your doctor.