Metanephrines, Fractionated, Quantitative, 24-Hour Urine

Urine (24-hour)

10 mL aliquot

2.5 mL aliquot (Note: This volume does not allow for repeat testing.)

Plastic urine container with no preservative or 6N HCl

No caffeine before or during collection. Monamine oxidase inhibitors should be discontinued at least one week prior to beginning collection.

Evaluation of catecholamine-secreting tumors of the adrenal medulla (pheochromocytomas) and extra-adrenal sympathetic and parasympathetic paragangliomas.

The low prevalence of PPGL combined with the limited diagnostic specificity of the metanephrine test means that false-positive results far outnumber true-positive results.^15,27 As outlined in the Endocrine Society Guidelines,¹⁵ all patients with positive test results should receive appropriate follow-up according to the extent of increased values and clinical presentation of the patient. The nature of this follow-up is a matter of clinical judgment based on the pretest probability of tumor and the extent and pattern of increases in test results.¹⁵

A number of medications and foods should be considered as potential causes of borderline elevations of plasma metanephrine and/or normetanephrine.^{2,5,27,28,32,33} These include:

• selective serotonin and norepinephrine reuptake inhibitors (SSNRIs)

• tricyclic antidepressants

• alpha blockers

• beta blockers

• monoamine oxidase inhibitors

• caffeine

• cigarette smoking

• strenuous exercise

Confirmatory testing after exclusion of these and other sources of false-positive results is often useful for ruling out disease.¹⁵

This test was developed and its performance characteristics determined by LabCorp. It has not been cleared or approved by the Food and Drug Administration.

Liquid chromatography/tandem mass spectrometry (LC/MS-MS)

Metanephrine and normetanephrine (together referred to as metanephrines) are the 3-methoxy metabolites of the catecholamines, epinephrine and norepinephrine, respectively. The methylation of catecholamines is accomplished by catecholamine O-methyltransferase, a membrane-bound enzyme of chromaffin cells.^1-8 Levels of these metabolites can be increased in both plasma and urine in patients with catecholamine-producing tumors such as pheochromocytomas, paragangliomas and neuroblastomas. Pheochromocytomas, intra-adrenal paraganglioma, and extra-adrenal sympathetic and parasympathetic paragangliomas (PPGLs) are rare neuroendocrine tumors derived from neural crest progenitor cells including adrenal chromaffin cells and similar cells in extra-adrenal sympathetic and parasympathetic paraganglia. Approximately 10% of pheochromocytomas and 35% of paragangliomas are malignant. About a third of these tumors are associated with three specific syndromes; von Hippel-Lindau syndrome, multiple endocrine neoplasia type 2 (MEN 2), and neurofibromatosis type 1. A number of germline mutations responsible for PPGLs have been identified.^9,10 Neuroblastomas are derived from immature embryonic neuroblast cells that also form tumors at adrenal and extra-adrenal locations, but present almost exclusively in childhood.¹¹

Patients with PPGLs can present with episodic hypertension related to excessive catecholamine synthesis and variety of other symptoms that can include tachycardia, headache, palpitations, profuse diaphoresis, and pallor.^5,12 Less frequently, these tumors can manifest as nausea, vomiting, flushing, and weight loss. In young patients with normal body weight, hypertension with diabetes mellitus may suggest PPGL.¹³ Many patients present with an unidentified mass lesion and no specific clinical symptoms associated with PPGL. Given the relative non-specificity of symptoms and the low prevalence of the condition (less than 1 per 100,000 individuals in the general population),¹⁴ it is not unusual for the diagnosis of PPGL to be delayed. The critical first step for diagnosis is to recognize the possibility of the tumor.^15-17 The consequences of delayed detection can be severe as excessive catecholamine secretion can precipitate life-threatening hypertension, intracerebral hemorrhage, and cardiac arrhythmias.^18,19 When detected early, these tumors are potentially curable.²⁰

Diagnosis of pheochromocytoma and paraganglioma relies on biochemical evidence of catecholamine production by the tumor. Guidelines suggest that measurement of plasma free metanephrines or urinary fractionated metanephrines should be performed in symptomatic patients,^15,22 patients with an adrenal incidentaloma,²¹ and in individuals who have a hereditary risk for developing a pheochromocytoma or paraganglioma.¹⁰ Metanephrines are produced continuously by the normal adrenal and by tumors via a process that is independent of catecholamine release, which for some tumors occurs at low rates or is episodic in nature.^6-8,15 While non-chromaffin cells of the sympathetic nervous system are the major sites of norepinephrine metabolism, they do not convert catecholamines to metanephrines because they lack the catecholamine O-methyltransferase enzyme. Consequently, levels of free metanephrines reflect functional chromaffin cell quantity and become elevated in patients with catecholamine producing chromaffin tumors.^1,6 Since many PPGLs produce and metabolize catecholamines but do not secrete the amines continuously or in amounts sufficient to produce a diagnostic signal, the metanephrines are superior to the parent catecholamines as diagnostic biomarkers.^23,24 The high diagnostic accuracy of measurements of urine fractionated metanephrines and plasma free metanephrines has been confirmed by a large number of studies.^{6-8,15,25-26,34}

Normetanephrine or metanephrine elevated 3-fold or more above upper cutoffs are rarely false positives and should be followed up in most cases by imaging to locate the tumor.^15,24,28 In cases of borderline elevation (less than 3-fold the upper limit of the reference interval), repeat testing and/or second-line tests such clonidine suppression test with measurements of plasma normetanephrine can be performed prior to proceeding to imaging studies.^15,29 Chromogranin A levels are elevated in most patients with PPGLs and have been associated with risk of malignancy.^34-35 However, the test is not specific and is seen in other disorders such as carcinoid.