Whole blood, serum or plasma
chloride is almost exclusively measured by potentiometry with an ion-selective
chloride electrode which utilizes membrane selectivity to
chloride ions. Other
anions such as
bromide,
iodide and thiosulphate can interfere but usually are not present in high enough concentration to cause significant cross reactivity. A patient from our
burns unit had serial
chloride measurements on a Radiometer ABL800 blood gas analyser. The results were higher in contrast to plasma measurements on the Abbott Architect Ci8200, which were within reference intervals and in line with the patient's pathophysiological status. This indicated a likely interference with the blood gas analyser
chloride estimation. The
chloride results on the ABL800 for 3rd, 4th and 5th day after the
burn accident were 170, 137 and 119 mmol/L. Corresponding plasma
chloride results on the Ci8200 were all around 105 mmol/L.
Nitrate was found to be markedly elevated in these samples, and the results were 6.7, 4.9 and 1.1 mmol/L, respectively (reference limit < 0.08 mmol/L). To further demonstrate
nitrate was the causative agent, pooled plasma spiked with 7 mmol/L of
sodium nitrate caused a rise in the ABL800
chloride from 105 to 202 mmol/L. Later we confirmed that the patient was topically medicated with
cerium nitrate cream (Flammacerium®, Sinclair IS Pharma, UK) for his
burns. In summary, the results clearly indicated
nitrate was the interferent with the ABL800
chloride estimation and the source was the topical
burns cerium nitrate cream.