Plasma free metanephrines is a relatively new test, and there is not yet widespread consensus on exactly how it should be used. Studies have shown that plasma testing is more sensitive than 24-hour urine catecholamines or metanephrines testing. However, this sensitivity brings with it a certain number of false positive results, especially when the test is ordered on patients in whom a pheochromocytoma is unlikely.  For this reason, plasma free metanephrines is not recommended as a routine test for the general public. Its primary use is in symptomatic patients when urine testing does not provide clear-cut results. Since both blood and urine test results may be affected by stress, caffeine, alcohol, and certain drugs, a doctor may investigate a positive result by evaluating a patient’s stresses and intake, alter or minimize these influences, and then repeat the tests to confirm the original findings.

Occasionally, the test may be ordered on an asymptomatic person if an adrenal tumor is detected during a scan that is done for another purpose or if the patient has a strong personal or family history of pheochromocytomas. These tumors may recur, and there is a genetic link in some cases.

The plasma free metanephrines test is primarily ordered when a doctor either suspects that a patient has a pheochromocytoma or wants to rule out the possibility. He may order it when a patient has hypertension and persistent or recurring symptoms, such as headaches, sweating, flushing, and rapid heart rate. It may also be ordered when a patient has hypertension that is not responding to treatment  as patients with a pheochromocytoma are frequently resistant to conventional therapies.

Occasionally, the test may be ordered when an adrenal tumor is detected incidentally or when a patient has a family history of pheochromocytomas. It may also be used as a monitoring tool when a patient has been treated for a previous pheochromocytoma.

Since the plasma free metanephrines test is very sensitive and pheochromocytomas are rare, a doctor may see more false positives with this test than true positives. The negative predictive value of the test, however, is very good. This means that if the concentrations of the free metanephrines are normal in the blood, then it is very unlikely that a patient has a pheochromocytoma.

If a symptomatic or asymptomatic patient has only moderately elevated free metanephrines, then a doctor may re-evaluate the patient’s medications, diet, and stress level to look for interfering substances. He may then re-test the patient, perhaps along with 24-hour urine catecholamine testing and/or metanephrine testing, to determine whether the free metanephrines are still elevated. If they are, then he may order imaging scans; if they are not, then it is unlikely that the patient has a pheochromocytoma.

If levels are elevated in a patient who has had a previous pheochromocytoma, it may be an indication that the tumor is recurring.

The majority of pheochromocytomas are found in the adrenal glands and most of the rest are found within the abdominal cavity. While the plasma free metanephrines test can help detect and diagnose pheochromocytomas, it cannot tell the doctor where the tumor is or whether it is benign or malignant (although most are benign).

An increased risk for developing pheochromocytomas can be inherited. MEN-1 and MEN-2 (Multiple Endocrine Neoplasia, types 1 and 2) syndromes are groups of conditions associated with alterations in specific genes. They increase the lifetime risk that those affected will develop tumors in one or more of their endocrine glands.

Several medications may interfere with the test. Consult your doctor as to which drugs you may safely discontinue before being tested. Also, caffeine containing foods such as coffee, soda and chocolate as well as cigarette and cigar smoking may interfere with the test and should be avoided prior to testing.