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Pseudohyponatraemia Lipids — ESENeph MCQ

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HardElectrolyte DisordersPseudohyponatraemia LipidsESENeph

A 35-year-old woman has a serum sodium of 142 mmol/L but measured serum osmolality is 280 mOsm/kg (low). Her lipid profile shows triglycerides of 45 mmol/L. What phenomenon explains the discrepancy?

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Correct answer: B — Pseudohyponatraemia – severe hypertriglyceridaemia displaces the aqueous phase, causing falsely low sodium measurement on indirect ion-selective electrode methods

Pseudohyponatraemia occurs when high concentrations of lipids (or proteins like in myeloma) displace the aqueous fraction of plasma. Indirect ion-selective electrode (ISE) methods (used in most laboratories) measure sodium in a diluted sample and calculate back – the lipid-rich phase dilutes the apparent sodium concentration. Direct ISE methods (used in blood gas analysers) and osmolality measurement (which reflects true solute concentration) are unaffected. True serum osmolality of 280 with Na 142 confirms that true sodium is normal – the measured sodium on the main analyser would have been falsely low.

Reference: Spasovski et al 2014 – Hyponatraemia Guideline; Dimeski 2010 – Pseudohyponatraemia