Free or total metanephrines for diagnosis of pheochromocytoma: what is the difference?

Pheochromocytomas, although a rare cause of hypertension, are dangerous tumors that require consideration in large numbers of patients. The resulting low prevalence of these tumors among the tested population and the inadequate sensitivity and specificity of commonly used biochemical tests make diagnosis of pheochromocytoma difficult and time-consuming. As outlined in two articles in this issue of the Journal (1, 2), measurements of plasma concentrations of metanephrines provide a promising, new, highly sensitive test for diagnosis of pheochromocytoma. A negative test result for plasma metanephrines means that a pheochromocytoma is highly unlikely so that no other tests are necessary (3, 4). Thus, measurements of plasma metanephrines provide a particularly good initial diagnostic test for exclusion of pheochromocytoma. The diagnostic superiority of plasma metanephrines over plasma or urinary catecholamines and urinary vanillylmandelic acid is clear (3–6). What remains unclear, as illustrated in the two articles in this issue of the Journal (1, 2), is whether measurements of plasma free (unconjugated) metanephrines or total metanephrines provide the better diagnostic test, and how these tests differ. Also unclear is how these tests differ from measurements of urinary metanephrines. Part of the confusion about tests of urinary total or fractionated metanephrines and plasma free, unconjugated, or total metanephrines stems from the unfortunate and confusing terminology used to distinguish these analytes. The term “metanephrines” describes two catecholamine metabolites: normetanephrine, the O-methylated metabolite of norepinephrine, and metanephrine, the O-methylated metabolite of epinephrine (Fig. 1). The term urinary “total” metanephrines was coined, based on historical precedents, to describe both normetanephrine and metanephrine measured together as a single concentration by early spectrophotometric assays. Spectrophotometric assays of urinary total metanephrines have been superseded by HPLC assays that allow separate measurement of normetanephrine and metanephrine, termed “fractionated” metanephrines. Despite the superiority of HPLC assays of plasma or urinary fractionated metanephrines over urinary total metanephrines (3–5, 7, 8), demand for the latter test persists. The delay of laboratories and clinicians to move to the better tests may partly reflect the confusing nature of the terminology used to distinguish the various assays. Adding to the confusion in terminology is the fact that the “free” or “unconjugated” metanephrines measured by Roden et al. (1 ) are different metabolites from the metanephrines commonly measured in urine or the plasma total metanephrines measured by Grouzmann et al. (2 ). Moreover, assays of plasma total metanephrines reflect fractionated measurements of normetanephrine and metanephrine; thus, these assays are not analogous to combined measurement of normetanephrine and metanephrine in assays of urinary total metanephrines. Plasma total metanephrines and urinary fractionated or total metanephrines are determined after urine or plasma samples are subjected to acid hydrolysis or enzymatic deconjugation with sulfatase. This liberates free metanephrines from the sulfate-conjugated metabolites. Plasma concentrations of total metanephrines (i.e., free plus conjugated metanephrines) are 20to 30-fold higher than free metanephrines and therefore largely reflect conjugated metanephrines, metabolites different from the free metanephrines. In assays of urinary metanephrines, the difference is even larger, with free metanephrines representing a small proportion (,3%) of the total measured as free plus conjugated metanephrines. Sulfate-conjugated metanephrines and catecholamines are formed from the free amines by the actions of a specific sulfotransferase isoenzyme, monoamine-preferring sulfotransferase (SULT1A3) (9 ). The presence of this sulfotransferase isoenzyme, and thus formation of sulfateconjugated metanephrines, in adrenal medullary chromaffin cells or pheochromocytoma tumor cells has not been established. Monoamine-preferring sulfotransferase is, however, found in high concentrations in the gastrointestinal tract, which is the source of most of the sulfateconjugated catecholamines produced in the body (10, 11).