Hyponatraemia is an electrolyte disturbance estimated to occur in approximately 30-35% of hospital inpatients [1]. Pseudohyponatremia is an important differential that occurs when there are significant levels of lipids or protein in plasma which alter the standard 93:7 ratio of water to solute[2].
A 62-year-old female presented with a 12-month history of painless obstructive jaundice. PET-CT scan identified an FDG-avid 43x41x34mm mass in the duodenum resulting in proximal common bile duct dilatation. Pathology demonstrated a cholestatic pattern with GGT 1,800U/L (RR<35), ALP 825U/L (RR 30-100) and bilirubin 410umol/L (RR<18). Electrolytes showed near isotonic hyponatremia with serum sodium 121mmol/L (RR 135-145) and serum osmolality 296mosm/kg (RR 275-295). Further investigations demonstrated elevated total cholesterol 29.8mmol/L (RR<6.0), triglycerides 3.9mmol/L (RR<2.0) and blood glucose 17.2mmol/L (RR 3.0-7.8), with normal total protein 64g/L (RR 60-80) and serum urea 4.3mmol/L (3.5-8.0). On examination, she did not have clinical features of hyponatraemia.
Paired serum and venous blood gas (VBG) sodium levels were performed. They showed a significant discordance with serum sodium 122mmol/L compared to VBG sodium 134mmol/L, thus confirming the presence of pseudohyponatraemia. VBG measures sodium directly and is not affected by changes in the solute levels in plasma in contrast to serum sodium which measures sodium via indirect means. Lipid electrophoresis and apolipoprotein B100 were assessed, and the results were highly suggestive of the presence of lipoprotein X, a lipid often secondary to cholestasis and a known cause of pseudohyponatraemia. She subsequently received a Whipple’s procedure, and there was an increase in serum sodium levels within 48 hours to 132mmol/L.
This case demonstrates lipoprotein X as the likely cause of pseudohyponatremia. VBG sodium should be measured when there is suspicion of pseudohyponatremia. The presence of lipoprotein X must be considered in all patients with cholestasis, and can be identified via lipid electrophoresis and measuring apolipoprotein B100.