Free Water Deficit (Hypernatremia)
Calculate the water needed to safely correct hypernatremia — without inducing cerebral edema.
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EasyClinic alerts when planned correction rates exceed safe thresholds, suggests appropriate fluid, and tracks lab trends in real time.
Also Known As
Formula
Free water deficit (L) = TBW factor × weight × (current Na / target Na − 1)
TBW factor: 0.6 (adult male, child), 0.5 (adult female, elderly male), 0.45 (elderly female).
Causes of hypernatremia
- Water loss: diabetes insipidus (central or nephrogenic), osmotic diuresis (DKA, mannitol), insensible losses (fever, burns), GI losses
- Reduced intake: elderly, debilitated, altered mental status (most common in India in summer)
- Sodium gain: hypertonic saline, NaHCO3, salt poisoning
Correction rules
- Chronic (>48h): max 10-12 mEq/L per 24 hours
- Acute (<48h, e.g. salt poisoning): can correct faster (1 mEq/L/hr initial)
- Fluid of choice: D5W (no Na) if euvolemic; 0.45% NaCl if hypovolemic
- Replace ongoing losses + insensible losses on top of deficit
- Recheck Na q4-6h, adjust rate
Frequently Asked Questions
▸D5W vs 0.45% NaCl — which to use?
D5W gives pure free water once dextrose is metabolised — preferred in euvolemic patients. 0.45% NaCl provides volume + free water — preferred in hypovolemic hypernatremia (e.g., post-osmotic diuresis).
▸Why is rapid correction dangerous?
Brain cells adapt to chronic hypernatremia by accumulating idiogenic osmoles. Rapid correction causes osmotic water shift INTO brain cells → cerebral edema, seizures, herniation. Always limit to 10 mEq/L per 24h in chronic cases.
▸Does the formula account for ongoing losses?
No — it only calculates the existing deficit. You must add ongoing urinary, GI, and insensible losses (~30-40 mL/hr in adults, more in fever) to maintenance.
▸What about hypernatremia in DI?
Identify type (central → desmopressin; nephrogenic → low-solute diet, thiazide, treat cause). The water deficit formula still applies, but treatment depends on the etiology — replacing water alone in untreated DI won't hold.
References
- Adrogue HJ, Madias NE. Hypernatremia. NEJM 2000;342(20):1493-9
- Sterns RH. Disorders of plasma sodium. NEJM 2015;372(1):55-65
- Indian Society of Nephrology Electrolyte Practice Recommendations
Smart electrolyte management in the EMR
EasyClinic tracks serum sodium trends, alerts on dangerous correction rates, and pre-fills fluid orders matched to the patient's deficit.
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