Falsely high urinary catecholamines induced by labetalol.

A preoperative patient with pheochromocytoma was satisfactorily treated with oral labetalol, while a conspicuous increase in urinary output of catecholamines (CA) and of vanillylmandelic acid (VMA) was observed after labetalol therapy. Exaggerated increases in urinary CA and in VMA were also confirmed in normal volunteers immediately after oral labetalol. These effects of labetalol on urinary CA and VMA, however, were proved to be largely due to the interference with the usual photometries rather than due to its stimulation of CA release. Therefore, it should be emphasized that clinical evaluations of CA and its metabolites must be performed before labetalol therapy to avoid an erroneous diagnosis of pheochromocytoma.