Sodium Deficiency (Hyponatremia): Test and Diagnosis

Laboratory parameters of the 1st order – obligatory laboratory tests.

  • Small blood count
  • Electrolytes – potassium, sodium [hyponatremia: < 135 mmol/l]
  • Urine sodium in spontaneous urine.
  • Total protein in serum (serum protein; serum protein).
  • Urinary and serum osmolality (U-osm, H-osm).
  • Glucose
  • Urea

Laboratory parameters 2nd order – depending on the results of the history, physical examination, etc. – for differential diagnostic clarification.

S-osm = 2 x S-Na+ + urea + glucose (concentrations in mmol/L) S-osm = 2 x S-Na+ + urea/2.8 + glucose/18 (urea and glucose in mg/dL).

Difference between calculated and measured serum osmolarity = osmotic gap [≤ 10 mosmol/L].

Procedure:

  1. If hyponatremia → exclude hypertonic hyponatremia: osmotic gap must be ≤ 10 mosmol/L
  2. Determination of urine sodium:
    • Hypovolemia: clin. Volume depletion (water loss):
      • Urine Na < 30 mmol/L = extrarenal cause.
      • Urine Na > 30 mmol/L = renal cause
    • Euvolemia: clin. Signs are usually not groundbreaking
      • Urine Na > 30 mmol/L
    • Hypervolemia: clin. Edema, heart failure, liver cirrhosis, nephrotic syndrome.
      • Urine Na < 20 mmol/L