A Negative Internal Standard in Urinary Metanephrines Quantification.

Abstract

A 67-year-old woman was sent to the cardiology department for a Staphylococcus aureus infective endocarditis treated with cefazolin. In view of her severe arterial hypertension resistant to triple therapy and the discovery of an adrenal nodule, urinary metanephrines were ordered. Quantification of metanephrines was performed by HPLC with electrochemical detection, following the instructions of the Chromsystems® Metanephrines in Urine kit. Instead of a positive peak for the internal standard, 4-hydroxy-3-methoxybenzylamine, a negative peak appeared in the patient’s chromatogram (Fig. 1), preventing the quantification of metanephrines. ... ... Interferences for urinary metanephrines determination mainly include a catecholamine-rich diet and drugs involved in catecholamine metabolism (1). Paracetamol and amoxicillin are also known to interfere with electrochemical detection (1, 2). 4-Hydroxy-3-methoxybenzylamine (in curry) (3) and methenamine (4) alter the internal standard signal. We determined here that cefazolin, which is mainly eliminated unchanged in urine and has electrochemical properties (5), turned the peak negative, which we confirmed by adding cefazolin to urine and water. If interference from cefazolin was suspected, one could use mass spectrometry or an alternative internal standard for accurate quantification.