Interference from Indian diet on the internal standard in a commercial method for the measurement of urinary metanephrines by high-performance liquid chromatography with electrochemical detection

Abstract

Introduction: Urinary metanephrines are widely used in the diagnosis of catecholamine secreting tumours. Over the past two years we have been using the commercial Recipe® ClinRep® Complete Kit for Metanephrines in Urine coupled with high-performance liquid chromatography and coulometric detection. It was noticed that the internal standards on the patient chromatograms were sporadically raised due to interference. Methods: The interference had identical chromatographic and electrochemical properties to the Recipe® internal standard (undisclosed identity). Inspection of the patient names showed it seemingly had a higher frequency and magnitude in patients of Indian origin. The source of the interference was tracked by dietary observation and intervention to curry leaves, a common component of Indian foods. Results: The interference was chromatographically and electrochemically indistinguishable from the internal standard. The mass spectrum of the pentafluoropropionate derivative of the interference matched the Recipe® internal standard and was identified as methoxyhydroxybenzylamine by library match. Conclusion: The component co-elutes exactly with internal standard and artifactually decreases the metanephrine and normetanephrine results. It is surprising that it has not been described previously. Patients being assessed for catecholamine secreting tumours should be advised to withdraw from eating Indian foods at least 24 h prior to commencement of urinary collection.