Serum indices can give valuable information and should be interpreted as a result. Lipaemia can influence results through different mechanisms, animportant one being the electrolyte exclusion effect. A case of pseudohyponatraemia due to this is reported. A 15-year-old female with type 2 diabeteswas seen for follow-up. Her biochemistry results revealed severe hyponatraemia of 118 mmol/L. Her capillary glucose concentration was 13.7mmol/L with a corrected sodium of 122 mmol/L. A lipaemic index of 3+ (absolute value 1320) was noted, which was not flagged by the laboratoryinformation system, as it was below the critical lipaemia limit for sodium determination. Repeated analysis of the same sample using a direct ionselective electrode method, the serum sodium concentration was 134 mmol/L (sodium corrected for glucose = 138 mmol/L). A triglyceride concentrationwas requested, which was severely raised (100.1 mmol/L). The electrolyte exclusion effect is an analytical phenomenon that causes falselylow electrolyte concentrations in the presence of severe lipaemia or hyperproteinaemia when using indirect analytical methods. These methods areused on many modern-day automated chemistry analysers and should be considered in a patient with asymptomatic hyponatraemia.