Metanephrines, Fractionated, Plasma Free

Plasma

0.6 mL

0.4 mL

Lavender-top (EDTA) tube

Patient should be fasting overnight (water and non-caffeinated soft drinks are permissible). Patient should avoid alcohol, coffee, tea, tobacco and strenuous exercise prior to collection. See Limitations section for more information about other potential causes of elevated metanephrines.

Evaluation of catecholamine-secreting tumors of the adrenal medulla (pheochromocytomas) and extra-adrenal sympathetic and para-sympathetic 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.^2,4 As outlined in the Endocrine Society Guidelines,² 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.²

Inappropriate sampling conditions,² as well as a number of medications and foods, should be considered as potential causes of borderline elevations of plasma metanephrine and/or normetanephrine.^1,5-9 These include:

• selective serotonin and norepinephrine reuptake inhibitors (SSNRIs)

• tricyclic antidepressants

• alpha 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.¹

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

Adult reference intervals applied to free metanephrines were originally developed by Eisenhofer and coworkers¹⁰ and further validated in subsequent studies.^11,12 Recognizing the potential deadly consequences of a missed diagnosis, cutoffs of reference were established to ensure optimum diagnostic sensitivity, with specificity a secondary consideration. Subjects fasted and abstained from caffeinated and decaffeinated beverages overnight. Plasma samples collected after at least 20 minutes of supine rest. For plasma metabolites, optimal performance was obtained using age-specific reference intervals for normetanephrine,^10,11 and for metanephrine single cut-offs for males and females that were slightly above 99.5 percentiles contributed to optimized performance.^11,12

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.^1,5,13-18 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 para-sympathetic paragangliomas (PPGLs) are rare neuroendocrine tumors derived from neural crest progenitor cells, including adrenal chromaffin cells and similar cells in extra-adrenal sympathetic and para-sympathetic 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.^19,20 Neuroblastomas are derived from immature embryonic neuroblast cells that also form tumors at adrenal and extra-adrenal locations, but present almost exclusively in childhood.²¹

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.^5,22 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.²³ 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 one per 100,000 individuals in the general population),²⁴ it is not unusualfor the diagnosis of PPGL to be delayed. The critical first step for diagnosis is to recognize the possibility of the tumor.^2,25,26 The consequences of delayed detection can be severe as excessive catecholamine secretion can precipitate life-threatening hypertension, intracerebral hemorrhage, and cardiac arrhythmias.^27,28 When detected early, these tumors are potentially curable.²⁹

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,^2,30 patients with an adrenal incidentaloma,³¹ and in individuals who have a hereditary risk for developing a pheochromocytoma or paraganglioma.²⁰ 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.^2,16-18 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, plasma levels of free metanephrines reflect functional chromaffin cell quantity and become elevated in patients with catecholamine-producing chromaffin tumors.^13,16 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.^32,33 The high diagnostic accuracy of measurements of urine fractionated metanephrines and plasma-free metanephrines has been confirmed by a large number of studies.^{2,10-12,16-18,34}

To ensure optimal diagnostic accuracy, samples for plasma metanephrine testing should be collected with the patient in a fully recumbent, supine position (for at least 30 minutes before sampling) and reference intervals established in the same position should be used.² Numerous studies have confirmed that lying supine at rest prior to blood collection prevents false-positive results due to postural-related stimulus of norepinephrine secretion.^{2,3,10,32,33,35,36}

Applying reference ranges established from samples collected in a supine position, the sensitivity of plasma metanephrines approaches 100%, such that a finding of normal levels has a very high negative predictive value for ruling out catecholamine secreting tumor.^2,36 Normetanephrine or metanephrine elevated three-fold or more above upper cutoffs are rarely false positives and should be followed up in most cases by imaging to locate the tumor.^2,6,33 In cases of borderline elevation (less than three-fold the upper limit of the reference interval) repeat testing with sampling in a supine position and/or second-line tests such as the measurement of fractionated 24-hour urinary metanephrines and performance of a clonidine suppression test with measurements of plasma normetanephrine can be performed prior to proceeding to imaging studies.^2,37 Chromogranin A levels are elevated in most patients with PPGLs and have been associated with risk of malignancy.^37-39 However, the test is not specific and is seen in other disorders such as carcinoid.