Free Water Deficit

Why use it

Essential for managing hypernatremia. Guides volume and rate of hypotonic fluid replacement. Prevents both under-correction and overly rapid correction (cerebral edema risk).

About

Calculates the estimated free water deficit in hypernatremic patients. This guides the volume of hypotonic fluid (D5W or half-normal saline) needed to correct serum sodium to normal levels. Correction must be done slowly to prevent cerebral edema.

When to use

• Hypernatremia (Na > 145 mEq/L)
• Dehydration with elevated sodium
• Planning free water replacement in ICU patients

Formula

Pearls & pitfalls

• This formula estimates ONLY the deficit — add ongoing losses (urine, insensible, GI).
• Rate of correction: ≤ 10-12 mEq/L per 24h for chronic (> 48h) hypernatremia.
• Acute hypernatremia (< 24h) can be corrected faster: 1-2 mEq/L per hour.
• Total body water (TBW) fraction varies: 0.6 (young males), 0.5 (elderly males, young females), 0.45 (elderly females).
• D5W is preferred for pure free water replacement; 0.45% NS provides half free water.
• Recheck sodium frequently during correction — adjust rate accordingly.

Next steps

Replace deficit over 48-72 hours. Do NOT correct faster than 10-12 mEq/L per 24 hours (chronic hypernatremia) or 1 mEq/L per hour (acute < 24h). Account for ongoing losses. Recheck Na every 4-6 hours.

Evidence

Based on fundamental physiology of body water distribution. Widely used in nephrology and critical care practice. Endorsed by UpToDate, clinical nephrology textbooks, and critical care guidelines.

References

1. Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499. — link