LAUSR

Emergency department presentations of sodium nitrate poisoning are increasing in frequency. Point-of-care blood gas analysis is useful for identifying methaemoglobinaemia and other abnormalities in such patients. Topically applied nitrate is known to positively interfere with chloride measurement in both point-of-care instruments and automated analysers of the clinical chemistry laboratory. In this article, the authors describe a case of pseudohyperchloraemia caused by sodium nitrate, which was consumed orally for the purpose of suicide. Consistent with the established pattern of interference, the ABL800 (Radiometer Medical, Brønshøj Copenhagen) blood gas analyser produced spuriously high chloride results, whilst the Alinity (Abbott Diagnostics, Abbot Park, Illinois) automated analyser resulted in chloride measurements comparable to those of inductively coupled mass spectrometry (ICP-MS). Both instruments, measure chloride with ion-selective electrodes (ISEs). The ABL800 (Radiometer) uses a membrane electrode, which is vulnerable to permeation by lipophilic nitrate ions, whereas the Alinity (Abbott) employs a silver chloride redox electrode system that is resistant to precipitation of silver nitrate due to its relatively high solubility. These mechanistic differences likely explain why nitrate interferes with some point-of-care devices but does not appear to affect the results of automated analysers.