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Free Water Deficit / Sodium Deficit

Dysnatraemia Correction Calculator
Calculates free water deficit for hypernatraemia correction and sodium deficit for hyponatraemia correction using hypertonic saline. Includes safe correction rate guidance to prevent cerebral oedema and osmotic demyelination syndrome.
Electrolytes Critical Care Nephrology Fluids
Dysnatraemia Calculator
Safe Correction Limit: Correction rate must NOT exceed 10 mEq/L per 24h (risk of cerebral oedema in hypernatraemia; osmotic demyelination syndrome in hyponatraemia). In chronic hyponatraemia: max 8 mEq/L per 24h.

Formulae

TBW = Weight (kg) × F Adult male: F = 0.60 Adult female: F = 0.50 Elderly male: F = 0.50 Elderly female: F = 0.45 Free Water Deficit = TBW × ([Na_measured / Na_target] − 1) Result in litres Sodium Deficit = TBW × (Na_target − Na_measured) [mEq] Volume of replacement fluid: 0.9% NS (154 mEq/L): Vol (mL) = Deficit / 154 × 1000 1.8% NaCl (308 mEq/L): Vol (mL) = Deficit / 308 × 1000 3% NaCl (513 mEq/L): Vol (mL) = Deficit / 513 × 1000

All sodium values in mEq/L. TBW = total body water. Free water deficit is the volume of pure water that must be replaced to normalise serum sodium in hypernatraemia.

Safe Correction Rates

ConditionSafe RateMax per 24hRisk of Rapid Correction
Hypernatraemia0.5 mEq/L/h8–10 mEq/L/dayCerebral oedema (rapid fall in tonicity causes cerebral swelling)
Acute symptomatic hyponatraemia1–2 mEq/L/h initially—Cerebral herniation if uncorrected
Chronic hyponatraemia0.5 mEq/L/h8 mEq/L/dayOsmotic demyelination syndrome (central pontine myelinolysis)

Fluid Composition Reference

FluidNa ContentUse
5% Dextrose (D5W)0 mEq/LFree water replacement (hypernatraemia)
0.45% NaCl77 mEq/LPartial free water + Na replacement
0.9% NaCl (NS)154 mEq/LIsotonic Na replacement
1.8% NaCl308 mEq/LHyponatraemia correction
3% NaCl513 mEq/LSymptomatic hyponatraemia; SIADH

Clinical Application

  • In hypernatraemia: use 5% dextrose or 0.45% NaCl for free water replacement — not plain water enterally in ventilated patients
  • In hyponatraemia: 3% NaCl reserved for symptomatic patients (seizures, coma, cerebral oedema)
  • DDAVP (desmopressin) 2 mcg IV may be used to "clamp" the sodium once target is reached in SIADH — prevents overshoot
  • Check serum sodium every 4–6h during active correction; adjust infusion rate accordingly
  • Urine sodium and osmolality guide differentiation of hyponatraemia aetiology (SIADH vs hypovolaemic)

References

  1. Adrogue HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493–1499.
  2. Adrogue HJ, Madias NE. Hyponatremia. N Engl J Med. 2000;342(21):1581–1589.
  3. Spasovski G, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Nephrol Dial Transplant. 2014;29(Suppl 2):i1–i39.

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