Free Water Deficit in Hypernatremia
Calculate free water deficit and maximum safe rehydration rate.
Switch how results are explained
Free water deficit is 4.5 L. Correct slowly: decrease serum Na⁺ by NO MORE than 8–10 mEq/L per 24 hours (approx. 0.5 mEq/L/hour) to avoid severe cerebral edema. Include ongoing obligate fluid losses in total fluid replacement volume.
- Evidence grade
- A
What it is and when to use it
Free water deficit estimates the volume of pure water required to lower elevated serum sodium back to a safe target level (typically 140 mEq/L) based on estimated total body water.
How to interpret it
Serum sodium should be corrected slowly (no more than 8–10 mEq/L per 24 hours) to prevent cerebral edema and permanent neurological damage.
Limitations and when not to use it
Always account for ongoing fluid losses (fever, diarrhea, urine) in total fluid orders. Recheck serum electrolytes frequently during active rehydration.
Formulas and cut-offs are from the original authors of each score; see the references.
Last reviewed: June 2026
References
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499. PMID:10816188