Specimen Name Result Units Range
Plasma Sodium 124 mmol/L (135-145)
Potassium 6.0 mmol/L (3.5-5.5)
Chloride 90 mmol/L (100-109)
Bicarbonate 12 mmol/L (22-32)
Glucose 6.5 mmol/L (3.8-5.5)
Urea 33 mmol/L (2.7-7.2)
Creatinine 290 μmol/L (60-110)
Urine Sodium 65 mmol/L (> 20)
Osmolality 310 mmol/kg

25yo male with a family history of renal failure.
Salt-Losing Nephritis - Polycystic Kidneys
Calculated osmolality: = 1.86*(Na+K) + urea + glucose + 10
= 1.86(124+6) + 33 + 6.5 + 10
≈ 291 mmol/L

Salt-losing nephritis can cause hypo- or hyper-osmolar (usually mild) hyponatraemia, depending upon elevation of urea.

Hyperosmolar hyponatraemia due to 'other' osmotic particles:

  • Glucose - DKA, HONC
    • corrected [Na+] ≈ measured + [BGL-10]/3
  • Urea - esp. salt-losing nephritis
    • high K+, creatinine & urea
  • Others - mannitol, glycine, alcohols