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Specificity of Biochemical Tests

As distinct from sensitivity, the specificity of a biochemical test indicates how likely it is that a patient with a positive test result (i.e., an elevated plasma concentration or urinary output) has a pheochromocytoma. Thus, a high sensitivity of one hundred percent indicates that the test reliably proves the presence of a pheochromocytoma in all patients with a positive test result. Importantly, it must be realized that a high specificity of 100% does not necessarilly mean that all patients with a pheochromocytoma will have a positive test result. This is a matter of sensitivity (see above).

Since upper reference limits of normal of most diagnostic tests are typically established from the 95% confidence intervals (with 2.5% below and 2.5% above) of a range of values determined in a reference population (see Table 1 for normal reference limits), it can be expected that all tests will give at least a 2.5% incidence of false-positive results (i.e., 2.5% of all tests results will show an elevated value that might suggest the presence of a tumor when none is really present). This means that no biochemical test can be expected to have 100% specificity and the best that might be expected would be 97.5%. In practice, however, the specificity of biochemical tests are often lower than 97.5%. This is particularly problematic for the biochemical tests used to detect pheochromocytoma.

Since the catecholamines and their metabolites are normally produced by sympathetic nerves and the adrenal medulla, none of these compounds are highly specific for the presence of a pheochromocytoma. In particular, plasma and urinary catecholamines may be elevated by a variety of physiological, pharmacological and pathological conditions (1). As yet, none of the various biochemical markers of pheochromocytoma have been shown by any rigorous study to be any more specific than the others for diagnosis of a tumor. Nevertheless, the combination of measurements of catecholamines and of certain metabolites can be useful in helping to distinguish elevated catecholamines due to a tumor from elevated catecholamines secondary to increased release from sympathetic nerves or the adrenal gland.

Increased release of catecholamines from sympathetic nerves or the adrenal gland secondary to exercise, mental stress, low blood pressure, low blood volume, low blood glucose or certain drugs are all conditions that may elevate plasma and urinary catecholamines and cause false positive test results. During blood sampling and 24 hr urine collections these influences must be avoided. Upright posture is another important determinant of catecholamine release, increasing plasma norepinephrine by as much as 3-fold above values in the lying position. Therefore, collection of blood samples should be performed with the patient resting quietly in the lying position for at least 20 minutes before sampling with an indwelling intravenous catheter previously inserted to avoid any possible acute stress associated with insertion of the needle.

A wide variety of pathological conditions may be associated with elevated plasma and urinary catecholamines. Congestive heart failure, renovascular hypertension, hypernoradrenergic hypertension, shock, sepsis, dumping syndrome, sleep apnea, anxiety neurosis and panic disorder are some of the syndromes that may be associated with elevated plasma or urinary catecholamines and clinical symptoms suggestive of a pheochromocytoma.

The above physiological and pathological conditions can also lead to elevated production of metanephrines and VMA, with attendant false-positive test results for these metabolites. However, because production of these metabolites is somewhat independent of catecholamine release from the sympathetic nerves or the adrenal medulla, their proportional increases are typically less than those in catecholamines. The metanephrines, in particular, are relatively poor markers of increased release of catecholamines from sympathetic nerves or the adrenal gland (33,41). Theoretically, this should make the metanephrines less prone to false-positive results in physiological and pathological states associated with sympatho-adrenalmedullary activation. However, there are other factors to consider that may additionally contribute to false positive test results for these tests. Deficiencies or pharmacological inhibition of MAO increases both urinary deconjugated and plasma free metanephrines due impaired breakdown of the metabolite by MAO and increased shunting of metabolism through O-methylation pathways (47,48). Severe renal failure can be particularly problematic in patients suspected of having a pheochromocytoma, making urine test results unreliable or urine collection impossible (49). In these patients biochemical diagnosis typically requires assays of plasma catecholamines and metanephrines. However, the sympathetic nerves can be activated in these patients resulting in elevated plasma norepinephrine concentrations. Also, end-products of catecholamine metabolism, that depend on elimination by the kidneys – such as the sulfate-conjugated metanephrines – tend to build up in plasma to very high levels (50). Reduced renal perfusion in states such as congestive heart failure and hypertension with impaired renal function can also reduce the circulatory clearance of metabolic end-products leading to increased plasma concentrations independent of any effect of sympathetic activation.

During interpretation of biochemical test results it is important to keep in mind that these are numerical values and should not be simply considered as simply either negative (within the normal range) or positive (above the normal range). Rather, the magnitude of an increase above normal values should also be considered. For example, a patient presenting with suggestive symptoms and a plasma norepinephrine concentration of over 2500 pg/mL (14.8 pmol/mL), approximately 5 times above the upper reference limit of normal, is far more likely to have a pheochromocytoma than a patient with the same symptoms and a plasma norepinephrine concentration just above the upper reference limits. The few conditions where plasma norepinephrine can reach such high levels (e.g., hypernoradrenergic hypertension, end stage congestive heart failure, circulatory shock) are easily excluded. In patients with pheochromocytoma, plasma concentrations of normetanephrine and metanephrine typically show much larger relative increases above the upper reference limits than observed for tests of catecholamines, urinary metanephrines and VMA. This indicates that at the higher limits more specific for a tumor, measurements of plasma normetanephrine and metanephrine provide better proof (i.e., are more specific) of a pheochromocytoma than other available tests.

Diagnostic Procedural Recommendations

Initial Biochemical Testing

With issues of sensitivity and specificity in mind, as well as consideration of the potential dangers of a pheochromocytoma and the rarity of the tumor, the most important consideration in choice of initial biochemical test is the reliability of the test for exclusion of pheochromocytoma. In pheochromocytoma, a missed diagnosis due to a false-negative test result can have catastrophic consequences for the patient. In contrast, a single false-positive test result can be refuted by further tests. Therefore, a suitably sensitive biochemical test remains the first choice in the initial work up of the patient suspected to be harboring a pheochromocytoma. With most available biochemical tests, however, confirming the absence of a tumor can be more problematic than confirming the presence of a tumor.

Because of the high sensitivity of the test, our recommendation is to use HPLC measurements of plasma free normetanephrine and metanephrine as the initial biochemical test of choice. This test should preferably be combined with HPLC measurements of plasma catecholamines collected under appropriately controlled circumstances for further interpretation of any elevations in plasma free metanephrines. To circumvent false-positive test results, appropriate consideration should always be given to interference with assay results by any drugs that the patient may be taking. In centers where measurements of plasma free metanephrines are not available, we recommend that initial biochemical tests for exclusion of a pheochromocytoma should include either HPLC measurements of plasma or 24 hr urine deconjugated (free + sulfate-conjugated) normetanephrine and metanephrine combined with measurements of plasma or urinary catecholamines. Because of a lower sensitivity, a finding of a normal 24 hr urinary output of VMA is of little practical use in initial exclusion of a pheochromocytoma.

If plasma free metanephrines have been run, and they are well within the normal range, then it is highly unlikely that the patient has anything but a very small tumor (less than 1 cm) and there should be little need to run further tests at this stage. On the other hand, normal results for each of the other biochemical tests, even when performed in combination, are still possible in a small percentage of patients with a pheochromocytoma. Thus, if plasma free metanephrines have not been run, but the other above test results are all normal, then it is still possible that the patient has a pheochromocytoma. If suggestive signs or symptoms persist, repeat biochemical testing may be appropriate.

Secondary Diagnostic Tests

If tests of plasma free metanephrines or any of the above combination of test results are positive then further tests must be considered to exclude or prove the existence of a pheochromocytoma. At this stage it is again important to consider any potential associated clinical conditions or influences of medications that may cause a false-positive test result (see above section on “Specificity”). Additional considerations include the magnitude of increase above normal of biochemical test results along with the pattern of alterations in biochemical parameters which may be used together to make some kind of qualitative assessment of the likelihood that the patient has a tumor. From this a strategy for further testing can be developed, including imaging studies to localize the tumor, provocation tests and further biochemical tests. Ultimately, it is a combination of biochemical and radiological tests combined with clinical assessment of symptoms, signs and associated conditions that provides the clinician with sufficient evidence to establish or refute the diagnosis of pheochromocytoma.

Ruling out Sympatho-adenalmedullary Activation

Use of the clonidine suppression test is particularly useful when there is suspicion that elevated plasma concentrations of norepinephrine are secondary to increased release of the transmitter from sympathetic nerves rather than from a pheochromocytoma. The basis of the test is simple and involves oral administration of the drug, clonidine, with measurements of plasma norepinephrine concentrations before and 3 hours after the drug. Clonidine acts on centers in the brain to decrease the release of norepinephrine from sympathetic nerve endings. The drug does not decrease release from a pheochromocytoma. Thus, in a patient with high plasma norepinephrine concentrations due to increased release of the transmitter from sympathetic nerves the drug will decrease the elevated norepinephrine concentrations and a pheochromocytoma can be excluded. In a patient with high plasma norepinephrine concentrations due to release from a pheochromocytoma, the high norepinephrine concentrations will remain unaffected and a pheochromocytoma becomes more likely.

Other indicators of sympatho-adrenalmedullary activation rather than a pheochromocytoma are the distict patterns of biochemical test results that accompany sympathetic activation as opposed to a tumor. Patterns of biochemical test results that are more suggestive of increased release from sympathetic nerves or the adrenal gland than from a tumor (such as occurs in hypernoradrenergic hypertension, renovascular hypertension, congestive heart failure, panic disorder, dumping syndrome, and other conditions), include proportionally larger elevations above the upper reference limits of normal of plasma norepinephrine or epinephrine than of plasma normetanephrine or metanephrine. In sympathetic activation, increases in plasma DHPG that parallel the increases in norepinephrine typically reflect a source of the elevated norepinephrine from sympathetic nerves rather than from a pheochromocytoma. This pattern of biochemical test results combined with a negative clonidine suppression test result (i.e., a substantial decrease in plasma norepinephrine after clonidine) is highly suggestive of sympathetic activation rather than a tumor, and, unless imaging studies suggest otherwise, a pheochromocytoma can be excluded and no further tests should be necessary.

Localizing the Tumor

If plasma or urine levels of catecholamines and metanephrines are high, suppression tests and/or stimulation tests are positive (i.e., plasma norepinephrine does not decrease after clonidine, plasma norepinephrine shows a substantial increase after glucagon), and if the biochemical pattern of test results are indicative of a tumor rather than increased release from sympathetic nerves or the adrenal gland, then it is important to localize the tumor for subsequent surgical removal.

In most cases of positive initial biochemical test results, where clinical suspicion of a pheochromocytoma remains reasonable, a CT or MRI scan of the entire abdomen may be immediately appropriate. A finding of an adrenal or abdominal mass together with suggestive clinical signs and symptoms and highly elevated catecholamines and their metabolites and an appropriate pattern of changes in metabolites and catecholamine precursors (i.e., larger relative increases in plasma free metanephrines than catecholamines and/or normal plasma DHPG levels) might be all that is needed at this stage to justify surgery. However, in more ambiguous cases it might also be appropriate to follow up with MIBG scintigraphy, preferably using the 123- iodine labeled compound rather than the 131-iodine labeled compound.

In rare cases, where imaging studies are all negative, but where suspicion of a pheochromocytoma remains high, it may be appropriate to consider a vena caval sampling procedure to establish the source of the high circulating levels of catecholamines or free metanephrines. In this procedure a catheter is threaded up the vena cava (normally from the major vein in the upper leg) allowing sampling of blood from various organs and tissues. A high concentration at one sampling site compared with others helps to localize the site responsible (i.e., the site of the tumor) for the elevated plasma concentrations of normetanephrine, metanephrine, norepinephrine and or epinephrine.

Identifying the “Silent” Pheochromocytoma

In occasional cases of sporadic pheochromocytoma and in some incidentalomas, but quite frequently during routine screening of patients with von-Hippel Lindau (VHL) disease or multiple endorcine neoplasia type 2 (MEN-2), a small adrenal mass may be present without any of the symptoms and signs associated with a pheochromocytoma (37,43,44). In these patients, the pheochromocytoma is “silent” and may not produce catecholamines in amounts sufficient to cause typical symptoms or a positive test result for plasma or urinary catecholamines, urinary VMA or urinary metanephrines. These patients, however, typically show some elevation of either plasma free normetanephrine (VHL patients) or both plasma free normetanephrine and metanephrine or only metanephrine (MEN-2 patients). Should this not be sufficient reason to operate, then it is important to follow these patients carefully over time, since as the tumor(s) enlarge plasma free normetanephrine and/or metanephrine increase providing stronger reason to operate. Use of glucagon-stimulation tests and other imaging techniques with higher specificity than CT, such as MIBG scintigraphy, may also be useful to identify a “silent” pheochromocytoma. However, the likelihood of false negative results for the glucagon-stimulation test and MIBG scanning, particularly in cases of small tumors, points to the need for careful follow-up studies in patients suspected of harboring a “silent” pheochromocytoma.


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This article has been adapted from other work by the authors and written for the express purpose of dissemination of relevant information to members of the Pheochromocytoma Support Group and all who visit the site. Pheochromocytoma is a rare tumor that is often misdiagnosed. It is therefore important that people with the tumor or suspected of harboring the tumor should be well informed with the most up-to-date information about available and most appropriate methods for diagnosis of the tumor.

This article written April, 1999.


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