Plasma free metanephrines is a test used to help diagnose or rule out the presence of a pheochromocytoma. Plasma free metanephrines 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 those 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 people 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 person's stresses, diet, and medications, 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 conducted for another purpose or if the person 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 someone has a pheochromocytoma or wants to rule out the possibility. He may order it when a person has hypertension and persistent or recurring symptoms, such as headaches, sweating, flushing, and rapid heart rate. It may also be ordered when a person has hypertension that is not responding to treatment as those with a pheochromocytoma are frequently resistant to conventional therapies.
Occasionally, the test may be ordered when an adrenal tumor is detected incidentally or when someone has a family history of pheochromocytomas. It may also be used as a monitoring tool when a person 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 metanephrine and normetanephrine are normal in the blood, then it is unlikely that a person has a pheochromocytoma.
If a symptomatic or asymptomatic person has only moderately elevated metanephrines, then a doctor may re-evaluate the person's medications, diet, and stress level to look for interfering substances. He may then re-test the person, perhaps along with 24-hour urine catecholamines testing and/or metanephrines testing, to determine whether the metanephrines are still elevated. If they are, then he may order imaging scans; if they are not, then it is unlikely that the person has a pheochromocytoma.
If levels are elevated in someone 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 the doctor as to which drugs may be safely discontinued 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.
This article was last reviewed on February 23, 2012. | This article was last modified on February 26, 2013.
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