Specimen Name Result Units Range
Plasma Sodium 121 mmol/L (135-145)
Potassium 4.6 mmol/L (3.5-5.5)
Chloride 90 mmol/L (100-109)
Bicarbonate 22 mmol/L (22-32)
Glucose 7.8 mmol/L (3.8-5.5)
Urea 3.5 mmol/L (2.7-7.2)
Creatinine 90 μmol/L (60-110)
Osmolality 263 mmol/kg (275-295)
Urine Sodium 6 mmol/L (> 20)
Osmolality 140 mmol/kg

Middle-aged patient post elective cholecystectomy.
Iatrogenic Water Overload - IVT with D5W

Hypo-osmolar hyponatraemia aetiology:

  1. Sodium loss (> water) - hypovolaemic
  2. Water excess - hypervolaemia (± oedema)
    • Excess hypotonic fluids - low-normal urea, hypotonic urine
    • CCF - high urea, hypertonic urine / low Na+
    • Nephrotic syndrome - high urea, isotonic urine
    • Cirrhosis - low urea
    • SIADH

However:

  • urine osmo should be << plasma and near maximally dilute (≈ 20-30 mmol/kg)
  • this suggests other stimulus to ADH secretion, e.g. post-operative, pain, drugs

 

DDx Hyponatraemia

  1. Iso-osmotic → factitious
    • hyperlipidaemia - usually only when plasma TG's > 50 mmol/l
    • hyperproteinaemia - multiple myeloma
    • IVT arm sample
  2. Hyper-osmotic → osmolar gap
    • hyperglycaemia → ↓[Na+] ≈ 1 mmol / 3 mmol ↑BGL
    • mannitol, glycine, glycerol, ? ± urea
    • other solutes not entering cells
  3. Hypo-osmotic
    • Hypovolaemic → persistent ADH effect
      • extrarenal losses - GIT, vomiting/diarrhoea, 3rd space
      • renal losses
        • diuretics, osmotic diuresis
        • salt losing nephritis
        • Addison's disease
        • heparin (aldosterone suppression)
      • fluid replacement deficient in Na+
    • Slightly hypervolaemic → fluid excess ≈ 3-4 l, no oedema
      • SIADH, reset osmostat
      • severe hypothyroidism, pituitary glucocorticoid deficiency
      • psychogenic polydipsia, inappropriate IV fluids
    • Hypervolaemic → fluid excess > ≈ 10 l, with oedema
      • 2° hyperaldosterone states
        • CCF
        • nephrotic syndrome
        • cirrhosis
      • renal failure