Hyponatraemia can be caused by a variety of reasons. Serum lipoprotein X is an abnormal lipoprotein rich in phospholipid and unesterified cholesterol. It can occur in patients with cholestasis resulting in pseudohyponatraemia when sodium is measured using a direct ion-specific electrode (ISE) method.
We present the case of a 60 year-old man who presented to hospital with painless jaundice. Bilirubin was 239umol/L (<20), conjugated bilirubin 148umol/L (<4), ALP 436U/L (30–100), GGT 1480U/L (<55), ALT 377U/L (<45), AST 245U/L (<35). Subsequent investigations including a computertomogram and liver biopsy confirmed metastatic cholangiocarcinoma. Comorbidities included hypertension, obesity, dyslipidaemia and type 2 diabetes mellitus. The patient was taking rosuvastatin, aspirin, atenolol, dapagliflozin, amlodipine, valsartan, hydrochlorothiazide and metformin.
Throughout the admission the patient was hyponatraemic. Paired testing confirmed a serum sodium 118mmol/L (135–145), calculated serum osmolality 260mmol/L (275-295), urine osmolality 243mmol/kg and urine sodium 23mmol/L. Renal function, cortisol, thyroid function, protein and glucose were unremarkable. He was clinically euvolaemic and displayed no neurological symptoms.
The thiazide was ceased without improvement of sodium levels. Measured osmolality 5 days later was 268mmol/kg with an osmolar gap of 14mmol/kg, serum sodium was 119mmol/L. A lipid profile to exclude pseudohyponatraemia showed an elevated cholesterol and LDL at 14.9 and 14mmol/L respectively, triglycerides 1.6mmol/L, HDL 0.2mmol/L, VLDL 0.7mmol/L, apolipoprotein A1 was low at 0.27g/L and apolipoprotein B normal at 1.11g/L. Lipid electrophoresis revealed lipoprotein X (figure 1). This was confirmed by the characteristic pattern on ALP isoenzyme electrophoresis (figure 2). Sodium was re-measured by direct ISE to correct for pseudohyponatraemia secondary to lipoprotein X and showed a sodium of 131mmol/L. Subsequent measurements by direct ISE showed normal sodium results. After stent placement and resolution of cholestasis, a repeated lipid profile 1 month later showed resolution of lipoprotein X.