Reduced specificity of the clonidine suppression test in patients with normal plasma catecholamine levels.

Although originally devised to discriminate between patients with pheochromocytoma and those with elevated plasma catecholamine levels for other reasons, the clonidine suppression test has recently been used in patients with normal resting catecholamine levels. Upon review of 49 patients evaluated for pheochromocytoma, 26 had elevated plasma norepinephrine levels and underwent clonidine suppression testing. Only one of 13 patients with sustained elevated norepinephrine levels before clonidine administration had a false-positive clonidine test result, in contrast to five of 13 patients whose pre-clonidine norepinephrine levels had decreased into the normal range. This difference was statistically significant (by chi-square) at p less than 0.05. Clonidine suppression testing for pheochromocytoma should be performed only in patients with elevated catecholamine levels, because it carries a higher false-positive rate in patients with normal resting norepinephrine levels; a more accurate diagnosis may be made in such patients using glucagon stimulation tests.