Plasma metanephrines

More specific laboratory tests are used to diagnose secondary hypertension. These include plasma norepinephrine and urinary metanephrine levels for pheochromocytoma, plasma and urinary aldosterone levels for primary aldosteronism, and plasma renin activity, captopril stimulation test, renal vein renins, and renal artery angiography for renovascular disease. 

Pheochromocytoma is a tumor of the adrenal medulla or sympathetic ganglion cells. The tumor secretes catecholamines, especially norepinephrine and epinephrine. The patient in the case study at the beginning of the chapter had a left adrenal pheochromocytoma that was identified by imaging. In addition, she had elevated plasma and urinary norepinephrine, epinephrine, and their metabolites, normetanephrine and metanephrine. 

Findings that none of the traditionally used biochemical tests could reliably detect aU cases of pheochromocytomas led to recommendations that biochemical testing should include a combination of measurements of catecholamines and catecholamine metabolites. VMA is excreted in urine in large amounts, which makes measurement of this metabolite a simple, easy to implement, and time-honored test for diagnosis of pheochromocytomas. Numerous studies have now made it clear, however, that measurements of urinary VMA provide a relatively insensitive diagnostic test with limited value for initial testing for pheochromocytomas. Therefore the usual recommendation has been that biochemical testing should include measurements of urinary or plasma catecholamines and urinary metanephrines. 

The basis for the high diagnostic efficacy of plasma free metanephrines is explained by several factors (1) plasma free metanephrines are produced by metabolism of catecholamines within pheochromocytomas, a process that occurs continuously and independently of variations in catecholamine release by tumors (2) normally only small amounts of metanephrines are produced in the body, and these are relatively unresponsive to sympathoadrenal activation compared with the parent amines and (3) VMA and the metanephrines commonly measured in urine are different metabofites from the free metanephrines measured in plasma, and are produced in different parts of the body by metabolic processes not directly related to the tumor itself." ... 

The high diagnostic sensitivity of measurements of plasma free or urinary fractionated normetanephrine and metanephrine makes these tests the most suitable choice for the initial work up of a patient with a suspected pheochromocytoma. Negative results by these tests virtually exclude a pheochromocytoma, whereas negative results by other tests do not. Exceptions include small or microscopic ([Pg.1047]

After the potential confounding influence of medications or other causes of false-positive results have been eliminated, some consideration should be given to the choice of additional biochemical tests and patterns of results necessary for more firmly establishing or refuting the diagnosis of a pheochromocytoma. When initial testing yields elevations in plasma normetanephrine, metanephrine, or both amines, this may be corroborated by a similar pattern of results after additional measurements of urinary normetanephrine and metanephrine. Conversely, when initial testing yields positive results for urinary fractionated metanephrines, additional measurements of plasma free metanephrines are useful. 

Patterns of increases in plasma free metanephrines and catecholamines can also be useful for confirming pheo-chromocytomas in patients in whom initial tests of free metanephrines are positive but msufficiently elevated for a firm diagnosis. More specifically, patients with a pheochromocytoma usually have larger relative increases in metanephrines than of the parent catecholammes, whereas patients with false-positive results caused by sympathoadrenal activation usually have larger increases in catecholamines than metanephrines. 

Additional markers of catecholamine overproduction have been employed to improve the biochemical detection of neuroblastomas. Free dopamine may be abnormal in urine from neuroblastoma patients with VMA and HVA excretion. Combined testing for VMA, HVA, and dopamine may therefore improve tumor detection, and in 1993 an international consensus report on neuroblastoma diagnosis added dopamine to the Hst of acceptable measurements to document the adrenergic nature of the tumor. Plasma measurements of dopamine and L-dopa, the amino acid precursor of dopamine, may also have clinical value and allow the alternate use of plasma. Measurement of methylated metabolites, especially normetanephrine, has also been explored. When urinary normetanephrine, metanephrine, methoxytyra-mine, dopamine, norepinephrine, VMA, and HVA were measured, clinical sensitivity for detection of neuroblastomas was 97% to 100% when results of normetanephrine testing were coupled either with VMA in the infants or with HVA in children greater than age 1. Even with an extended panel of catecholamines and metabolite measurements, a low incidence of nonsecreting tumors continues to be identified and should be considered in the interpretation of a negative test result. 

Isolated deficiencies of MAO A and B are extremely rare and are associated with distinct clinical and neurochemical phenotypes. Deficiency of MAO A is associated with a behavioral disorder characterized by increased aggressiveness. Plasma and urinary levels of deaminated metabolites of catecholamines are severely decreased, whereas levels of normetanephrine and metanephrine are increased. An increased ratio of plasma normetanephrine to DHPG has therefore been proposed to provide a sensitive marker for the deficiency state. In contrast, deficiency of MAO B is associated with a mild phenotype, the only biochemical alteration is increased urinary excretion of phenylethyiamine. 

Interpretation of a biochemical test result as normal or abnormal depends on availability of valid reference intervals (see Chapter 16). For tests of a single analyte, such as VMA, it can be expected that at least 2.5% of patients without pheochromocytomas will have values for the analyte above the upper reference limit and 2.5% below the lower reference limit. Up to a 5% incidence of false-positive results might be expected for tests of pairs of analytes, such as norepinephrine and epinephrine in tests of urinary or plasma catecholamines or normetanephrine and metanephrine in tests of plasma free or urinary fractionated metanephrines. False-positive rates usually, however, tend to be higher than expected this is likely due to reduced control over sampling conditions and sources of interference or differences in clinical characteristics of reference and patient populations. 

Use of appropriately matched reference populations can be important for effective diagnosis of monoamine-producing tumors among different populations of patients tested for such tumors. Urinary and plasma levels of catecholamines and metanephrines show different ranges in hypertensives or hospitalized patients compared with norraotensive healthy volunteers, children compared... 

Reference intervals for plasma and urinary catecholamines and catecholamine metabolites also differ according to sex and age. Females have lower plasma concentrations of epinephrine and metanephrine than males. Similarly, 24-hour urinary outputs of catecholamines and metanephrines are lower in women than men for epinephrine this difference remains significant when values are normalized for creatinine excretion Plasma levels of norepinephrine and normetanephrine increase with advancing age in adults, whereas plasma levels of epinephrine and metanephrine are little affected. Age-related increases in 24-hour urinary outputs of norepinephrine and normetanephrine have also been reported,but not consistently by all studies. In general, the influences of age... 

The free 0-methylated amine metabolites are present in plasma at picomolar concentrations that have made their accurate measurement technically difficult. Measurements of plasma metanephrines therefore represent relatively recent developments. The.first method enabling accurate measurement of plasma free normetanephrine involved a radioenzymatic assay in which normetanephrine was converted to H-iabeled metanephrine using preparations of the enzyme phenylethanolamine-N-methyltransferase, incubated with H-methyi-labeled S-adenosylmethionine. This method, however, did not allow measurements of metanephrine or methoxytyramine, and therefore had limited clinical utility. 

The first HPLC methods for measuring plasma metanephrines in the early 1990s featured an acid-hydrolysis step similar to that used for routine measurements of urinary metanephrines. These measurements of plasma deconjugated (free plus conjugated) metanephrines indicated promise for diagnosis of pheochromocytomas. Very high levels of the deconjugated metabolites were also found in patients with renal failure. 

An HPLC method for the more difficult measurement of plasma free metanephrines was first described in 1993. This method, like those involving measurements of plasma or urinary deconjugated metanephrines, requires a preana-lytical cation-exchange extraction and purification step. The low plasma concentrations of free metanephrines present several technical challenges. In particular, low levels of interfering substances, such as acetaminophen, tend to be more troublesome to measurements of plasma concentrations of the free metabolites than to the higher decon-... 

TABLE 29 8 Reference Intervals for Plasma Free and Deconjugated Metanephrines in Normotensive and Hypertensive Adults and in Normotensive Children... 

Eisenhofer G, Reiser H, Friberg P, Mezey E, Huynh TT, Hiremagalur B, et al. Plasma metanephrines are markers of pheochromocytoma produced by catechoi-O-methyltransferase within tumors. J CUn Endocrinol Metab 1998 83 2175-85. 

Eisenhofer G, Lenders JW, Linehan WM, Walther MM, Goldstein DS, Reiser HR. Plasma normetanephrine and metanephrine for detecting pheochromocytoma in von Hippel-Lindau disease and multiple endocrine neoplasia type 2. N Engl J Med 1999 340 1872-9. 

Goldstein DS, Holmes C, Sharabi Y, Brentzel S, Eisenhofer G. Plasma levels of catechols and metanephrines in neurogenic orthostatic hypotension. Neurology 2003 60 1327-32. 

Lenders JW, Eisenhofer G, Armando I, Keiser HR, Goldstein DS, Kopin IJ. Determination of metanephrines in plasma by liquid chromatography with electrochemical detection. CHn Chem 1993 39 97-103. 

Lenders JW, Keiser HR, Goldstein DS, WiUemsen JJ, Friberg P, Jacobs MC, et al. Plasma metanephrines in the diagnosis of pheochromocytoma. Ann Intern Med 1995 123 101-9. 

Raber W, Raffesberg W, Bischof M, Scheuba C, Niederle B, Gasic S, et al. Diagnostic efficacy of rmconjugated plasma metanephrines for the detection of pheochromocytoma. Arch Intern Med 2000 160 2957-63. 

Roden M, Raffesberg W, Raber W, Bernroider E, Niederle B, Waldhausl W, Gasic S. Quantification of unconjugated metanephrines in human plasma without interference by acetaminophen. Clin Chem 2001 47 1061-7. 

Sawka AM, Jaeschke R, Singh RJ, Young WF, Jr. A comparison of biochemical tests for pheochromocytoma measurement of fractionated plasma metanephrines compared with the combination of 24-hour urinary metanephrines and catecholamines,... 

Weise M, Merke DP, Pacak K, Walther MM, Eisenhofer G. Utility of plasma free metanephrines for detecting childhood pheochromocytoma. 

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