Free Water Deficit in Hypernatremia

Calculates the free water deficit to guide correction of hypernatremia. Determines how much free water is needed to lower serum sodium to target level.

kg
Current body weight in kg.
Total body water fraction: 0.5 for females/older adults, 0.6 for males.
mEq/L
Current serum sodium concentration.
mEq/L
Desired target sodium. Usually correct gradually - not more than 12 mEq/L per 24 hours.

References

  1. Hypernatremia - StatPearls — NCBI Bookshelf/NIH (2023)

Note: The 4 mL/kg formula estimates the amount of fluid needed to change serum sodium by 1 mEq/L. TBW fraction varies: 0.5 for females/elderly, 0.6 for males. For children, use 0.6. In severe malnutrition or edema, TBW estimates may be inaccurate.

Warnings & Limitations

  • DO NOT correct faster than 12 mEq/L in 24 hours (max 0.5 mEq/L per hour).
  • Rapid correction can cause cerebral edema, seizures, and death.
  • In volume-depleted patients, first resuscitate with isotonic fluids, then correct free water deficit.
  • In sodium intoxication, free water requirement may cause volume overload - may need loop diuretics plus D5W.
  • Ongoing losses (osmotic diuresis, diabetes insipidus) must be replaced continuously.