Bedside Rounds

Free water deficit for hypernatremia

Estimates how much free water is needed to bring sodium back to 140 mEq/L in hypernatremia, using total body water adjusted for age and sex.

What goes into the Free water deficit

  • Sodium (mEq/L)
  • Weight (kg)
  • Patient: Adult male (TBW 0.6); Adult female (TBW 0.5); Older male (TBW 0.5); Older female (TBW 0.45)

How to interpret the result

  • Free water deficit = total body water × (Na ÷ 140 − 1)
  • Total body water: 0.6 × weight (young men), 0.5 (young women and older men), 0.45 (older women)
  • Chronic hypernatremia: lower sodium by no more than about 10 mEq/L in 24 hours

Clinical pearls

  • Add ongoing losses (urine, GI, insensible) to the deficit.
  • Recheck sodium every 4–6 hours while correcting.

Evidence

  1. Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342:1493-1499.

Related workups

Delirium / acute confusionAcute kidney injuryHypernatremiaHyperosmolar hyperglycemic state (HHS)

Related calculators

Sodium corrected for glucoseAnion gap and delta ratioAll calculators

For education only. Check the inputs and the population the score was validated in before acting on it. Not a substitute for clinical judgment.