[Absence of labetalol interference on urine metanephrine determination in hypertensive patients]

The dosage of urinary catecholamines and their metabolites is a main element of diagnosis in the research of a pheochromocytoma in patients with high blood pressure. The literature reports high values of these compounds in patients treated with labetalol (an alpha/beta-blocker). An analytical interference has been evoked to explain these misleading results, which have not been observed with other beta-blockers. The goal of this work was to look for this eventual analytical interference in the dosage of urinary metanephrine by reversed phase liquid chromatography coupled with electrochemical detection, in patients with high blood pressure. Eighteen hypertensive patients, 52 +/- 14 years old, were included in the study. In 8 patients, a dosage of metanephrine, normetanephrine and creatinine on a 24 hours urine sample was performed before (D1) and 24 hours after (D3) the prescription of labetalol (200 mg twice a day). In the other group, labetalol was not prescribed but dosage was made in the same conditions. Urinary excretion of these compounds (metanephrine+normetanephrine) divided by urinary creatinine was not modified in the treated group (0.16 +/- 0.08 vs 0.14 +/- 0.04), nor in the reference group (0.17 +/- 0.08 vs 0.17 +/- 0.08). This study shows that administration of labetalol in patients with essential hypertension does not interfere with urinary metanephrine and normetanephrine determination after 48 hours of treatment. This implies that research for a pheochromocytoma is possible in patients with hypertension and receiving labetalol, by using reversed phase liquid chromatography coupled with an electrochemical detector for the dosage of urinary metanephrine and normetanephrine.