The clonidine test for the diagnosis of pheochromocytoma: the usefulness of urinary metanephrine measurements.

1. The clonidine suppression of urinary metanephrines as a criterion for the diagnosis of pheochromocytoma is described. Twenty-four patients were divided into 3 groups: Group I, 10 patients with pheochromocytoma (confirmed by tomography and surgery); Group II, 9 patients with suspected pheochromocytoma (clinical evidence plus one mildly elevated value of urinary metanephrines, but with negative tomography); Group III, 5 patients with proven essential hypertension. 2. Urinary metanephrine levels were determined in urine collected before (basal) and 3 h after a single oral dose of clonidine (0.4 or 0.8 mg). 3. Mean basal urinary metanephrine levels were above normal for group I (9.2 +/- 2.2 micrograms/mg creatinine) and group II (2.2 +/- 0.3 micrograms/mg creatinine) but were within the normal range for group III (0.6 +/- 0.1 microgram/mg creatinine). After clonidine administration, urinary metanephrine levels remained elevated for all patients with pheochromocytoma but decreased to within the normal range for all but one patient in group II. The urinary metanephrine levels of group III were not significantly altered by clonidine. 4. These data demonstrate that, when monitored by the clonidine suppression test, urinary metanephrine levels are useful for the diagnosis of pheochromocytoma, permitting the differentiation of affected patients from those exhibiting essential hypertension and increased sympathetic drive.