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Abstract: PO1439

Hyponatremia: Mind the (Osmolar) Gap

Session Information

Category: Trainee Case Report

  • 902 Fluid, Electrolyte, and Acid-Base Disorders: Clinical


  • Ayyoub, Joy, Penn Medicine, Philadelphia, Pennsylvania, United States
  • Zonoozi, Shahrzad, Penn Medicine, Philadelphia, Pennsylvania, United States
  • Alborzi, Pooneh, Penn Medicine, Philadelphia, Pennsylvania, United States

Hyponatremia, one of the most commonly encountered electrolyte abnormalities, is associated with considerable mortality and morbidity. It is important to rule out pseudohyponatremia by determining serum tonicity.

Case Description

65-year-old female with history of hypertension presented with worsening painless jaundice. Initial investigation was notable for an obstructive liver injury; total bilirubin of 28.4 mg/dL (direct 19.5 mg/dL and indirect 8.9 mg/dL), ALP 1225 U/L, AST 237 U/L, ALT 384 U/L and GGT 2274 U/L. She was also found to have a sodium of 126 mmol/L and potassium of 2.5 mmol/L.

With fluids and potassium repletion, her sodium plateaued at 131 mmol/L. Further investigation revealed a measured serum osmolality of 301 mOsm/kg with an osmolar gap of 33 mOsm/kg, and a urine osmolality of 589 mOsm/kg. Sodium analysis using ion-selective electrode (ISE) showed a correction in the sodium from 131 mmol/dL to 139 mmol/L on the same specimen, confirming the diagnosis of pseudohyponatremia. Lipid panel showed severe hypercholesterolemia (total cholesterol 1016 mg/dL, LDL 868 mg/dL, HDL 31 mg/dL and triglycerides at 604 mg/dL).

Patient underwent endoscopic retrograde cholangiopancreatography and biliary sphincterotomy with biopsy consistent with adenocarcinoma of the pancreas. Following sphincterotomy, lipid panel and serum sodium normalized without further intervention.


Serum cholesterol is elevated in cholestasis because its metabolic degredation and excretion are impaired. Much of the cholesterol is in the form of lipoprotein-X, an abnormal lipoprotein observed only in patients with cholestasis.

Standard methods of sodium analysis, indirect ISE, calculates electrolyte concentration on the assumption that the non-aqueous portion of serum, predominantly proteins and lipids, comprises approximately 7% of a patient's plasma volume. In our patient with significant hyperlipidemia, this led to falsely low indirect ISE values. Direct potentiometric measurements use undiluted samples and are not subjected to this artifact, a method also used in blood gas analysis.

This case demonstrates a rare presentation of pseudohyponatremia and highlights the importance of its consideration in cases where the serum osmolarity is normal or when an osmolar gap is present suggesting reduced plasma water content or the presence of ineffective osmoles.