Close the gap

Abstract

Background: Serum chloride (Cl-) is measured via direct or indirect ion selective electrodes in most laboratories. There have been reports of falsely elevated Cl- concentrations and a resultant negative anion gap due to interference from drugs containing bromide, which is a negatively charged halide, similar to Cl-.1,2 Case: A 1-year-old girl with severe, treatment resistant epilepsy presented for routine bloods. Her medication history included multiple anti-epileptics including phenobarbitone, clobazam, topiramate, vigabatrin, and also potassium bromide. No abnormality was found on full blood count, thyroid functions tests, liver and kidney function tests apart from a mildly elevated GGT of 30 U/L (RI 5–20 U/L). The anion gap was negative as Cl- was repeatedly unmeasurably high (>150 mmol/L), with the remainder of the electrolytes within normal limits. Review of her medication history prompted the possibility of bromide interference. We measured Cl- and bromide via ICP-MS: Cl- was 69 mmol/L and bromide markedly elevated at 35 mmol/L. Conclusion: Bromide-containing medications can cause a negative anion gap as a result of a falsely elevated chloride concentration. When this is suspected, measurement of chloride via a method different to ISE should be considered.