Amoxicillin-associated interference in an HPLC-EC assay for urinary fractionated metanephrines: potential pitfall in pheochromocytoma biochemical diagnosis.

Measurement of urinary fractionated metanephrines represents a first-line test for the biochemical diagnosis of pheochromocytoma. The high performance liquid chromatography coupled to electrochemical detection (HPLC-EC) assays used in the routine clinical laboratory can be subjected to analytical interferences by the presence of drugs or their metabolites. In this paper we describe the interference on urinary normetanephrine (uNMN) caused by amoxicillin. Two pediatric patients suspected of pheochromocytoma had very high uNMN levels (2543 and 4227μg/g Cr respectively; upper reference value: 339μg/g Cr). Amoxicillin interference was assessed by comparison for co-elution with uNMN and by LC-MS/MS analysis. After amoxicillin interference was suspected and the therapy was stopped uNMN levels returned to normal (149 and 214μg/g Cr respectively). Chromatograms obtained by HPLC-EC clearly showed that amoxicillin co-elutes with uNMN. Patients' uNMN levels measured by LC-MS/MS were in the normal range. Amoxicillin is responsible for analytical interference on HPLC-EC assay for uNMN. This finding can be of help in distinguishing true-positive from false-positive results in the course of a biochemical diagnosis for pheochromocytoma.