Just How Common is Pseudohyponatraemia? Retrospective Survey of 1,041 Patients with Serum Sodium 125 mmol/L or Less

Introduction: Hyponatraemia is usually associated with a low serum osmolality <280 mOsm/kg. Iso-osmolar (serum osmolality 280-300mOsm/kg) or hyperosmolar (serum osmolality >300 mOsm/kg) hyponatraemia should alert the clinician to one of two possibilities: pseudohyponatraemia, a laboratory artefact that occurs when patients with severe hyperlipidaemia or severe hyperproteinaemia have their sodium checked using indirect ion selective electrodes; or dilutional hyponatraemia which reflects the presence of solutes that increase osmolality such as glucose and alcohol, causing water to move from the intracellular to extracellular space. Objectives: To assess the prevalence and aetiology of pseudohyponatraemia in an unselected series of patients in southwest Scotland; and to determine the other causes of iso and hyperosmolar hyponatraemia in this patient population. Design: Retrospective cohort study. Setting: Southwest Scotland. Participants: 84,108 patients who had serum urea and electrolytes measured during an 18-month period from August 2023 to January 2025. Main Outcome Measures: Serum sodium, serum osmolality, serum lipids and serum immunoglobulins. Results: 1,041(1.2%) patients had serum sodium 125 mmol/L or less. Iso-osmolar and hyperosmolar hyponatraemia accounted for 33(4%) of 824 patients who had simultaneous measurements of serum osmolality. We identified one probable case of pseudohyponatraemia due to hyperlipidaemia in this group. Alcohol excess and hyperglycaemia were the likely causes of dilutional hyponatraemia in eight and four patients respectively. We further identified one possible case of pseudohyponatraemia among the 217 patients who did not have their serum osmolality measured. Conclusions: Despite its rarity, the recognition of pseudohyponatraemia remains important because it is a laboratory artefact for which no specific treatment is required.