The plasma free metanephrines test is used to help diagnose or rule out the presence of a rare tumor called a pheochromocytoma or a paraganglioma that releases excess metanephrines. Testing also may be used when a tumor has been treated or removed to monitor for recurrence.
Metanephrine and normetanephrine are breakdown products (metabolites) of epinephrine (adrenaline) and norepinephrine. Epinephrine and norepinephrine are hormones called catecholamines that are released into the blood in response to physical or emotional stress and help regulate the flow and pressure of blood throughout the body. (For more on this, see the "What is being tested?" section.) Since pheochromocytomas and paragangliomas produce these hormones in excess, measuring the amount in the blood and/or urine may help detect the tumor.
The Endocrine Society recommends using a test for plasma free metanephrines or urine metanephrines to evaluate an individual for a pheochromocytomas or paragangliomas. The blood test may be most useful when the person has persistent hypertension or is currently experiencing a sudden burst (paroxysm) of hypertension. This is because the hormones do not stay long in the blood; they are used by the body and rapidly broken down (metabolized) and/or then eliminated. Urine testing measures the total amount of metanephrines released into the urine in 24 hours. Since the hormone levels may fluctuate significantly during this period, the urine test may detect excess production that is missed with the blood test.
The test for metanephrines is very sensitive and false positives do occur. The test is affected by stress, drugs, smoking and various foods such as caffeine-containing drinks and alcohol. If a person has only moderately elevated metanephrines, then the healthcare provider may re-evaluate the person's medications, diet, and stress level to look for interfering substances or conditions. Once these are resolved, the person may be tested again to determine whether the metanephrines are still elevated and take appropriate action. Sometimes the plasma free metanephrines test will done in conjunction with 24-hour urine metanephrines testing and/or followed by catecholamines testing to help decide if a catecholamine-secreting tumor is present.
The plasma free metanephrines test is primarily ordered when a health practitioner either suspects that someone has a catecholamine-secreting tumor or wants to rule out the possibility. It may be ordered when a person (especially a person younger than age 40) has sudden bursts (paroxysms) of signs and symptoms, such as:
High blood pressure (hypertension), especially when a person has hypertension that is not responding to treatment since people with catecholamine-secreting tumors are frequently resistant to conventional therapies
Rapid heart rate (palpitations)
Testing may also be ordered when a catecholamine-secreting tumor has been treated or removed to monitor for recurrence.
Occasionally, the test may be ordered when an adrenal tumor is detected incidentally, such as during a scan conducted for another purpose, or when someone has a strong personal or family history of catecholamine-secreting tumors. It may also be ordered for an asymptomatic person with a known genetic risk of these tumors.
Normal metanephrine and normetanephrine blood levels mean it is unlikely that the person tested has a catecholamine-secreting tumor. The plasma free metanephrines test is very sensitive and these tumors are rare, so the negative predictive value of the test is very good.
High levels of metanephrine and normetanephrine in a person with signs and symptoms suggest the presence of a catecholamine-secreting tumor. It is recommended that patients with a positive result should receive appropriate follow-up according to the test values noted and the clinical presentation. Imaging scans, such as MRI, should be done to locate the tumor.
If levels are elevated in someone who has been treated for a catecholamine-secreting tumor, it may be an indication that the treatment was not fully effective or that the tumor is recurring and appropriate follow-up is needed.
While the plasma free metanephrines test can help detect and diagnose the presence of a catecholamine-secreting tumor, it cannot tell the healthcare provider the location of the tumor or whether it is benign or malignant (although most are benign). Imaging studies may be used to locate the tumor and genetic testing may be used to determine potential malignancy.
It has been proposed that all patients diagnosed with a pheochromocytoma or extra-adrenal paraganglioma should consider genetic counseling and testing. Early identification of a hereditary syndrome allows for early screening for other associated tumors. In addition, some people with a hereditary syndrome are more likely to develop malignant or recurrent disease. Knowledge of the specific genetic mutation permits increased vigilance during preoperative localization or postoperative surveillance of such patients.
It is also recommended that if a mutation is identified, predictive genetic testing should be offered to asymptomatic at-risk family members.
A variety of medications can interfere with catecholamines testing. However, it is important to talk to your healthcare provider before discontinuing any prescribed medications. The practitioner will work with you to identify potentially interfering substances and drug treatments and to determine which of them can be safely interrupted and which must be continued for your well-being.
Tests used for determining the presence of catecholamine-secreting tumors (pheochromocytomas and paragangliomas) include plasma catecholamines, urine catecholamines, plasma free metanephrines and urine metanephrines. These tests measure either the catecholamines or their metabolites (metanephrines) and have varying sensitivity and specificity. Current guidelines recommend plasma free metanephrines or urinary fractionated metanephrines for initial biochemical testing. The healthcare provider may, however, select any one (or more than one) depending on the person’s presentation, family history, and/or genetic profile.
This article was last reviewed on October 9, 2015. | This article was last modified on October 9, 2015.
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