Laboratory & Electrolytes

Free Water Deficit

Calculate free water deficit in hypernatraemia to guide safe rehydration therapy.

When to use: Calculate free water deficit whenever serum sodium exceeds 145 mEq/L (hypernatraemia) to guide the volume and rate of rehydration. Hypernatraemia most commonly results from inadequate water intake (elderly, ICU patients on nil per oral), diabetes insipidus (central or nephrogenic), or excess sodium administration. Replace the deficit with free water (oral or 5% dextrose IV) over 48–72 hours, aiming to lower sodium by no more than 10 mEq/L per day to prevent cerebral oedema from rapid osmotic shifts.

Calculator

Body Weight(kg)

Current Serum Sodium(mEq/L)This formula is intended for hypernatraemia (Na >145 mEq/L)

Patient CategoryTBW fraction: male 60%, female 50%, elderly female 45%

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Formula

Free Water Deficit (L) = TBW × (Serum Na / 140 − 1) TBW = Weight × [0.6 (male) | 0.5 (female) | 0.45 (elderly female)] <1 L: Mild (green) | 1–3 L: Moderate (amber) | >3 L: Severe (red) Safe correction: ≤10 mEq/L/day to prevent cerebral oedema

Key Points for NEET PG

Hypernatraemia definition: serum Na >145 mEq/L; severe: >160 mEq/L — neurological symptoms (lethargy, seizures, coma).
Most common cause of hypernatraemia in hospitalised adults: inadequate free water replacement (insensible losses + fever + inadequate IV fluids).
Diabetes insipidus (DI): central DI (ADH deficiency — treat with desmopressin); nephrogenic DI (ADH resistance — treat with low-sodium diet ± hydrochlorothiazide).
Correction rate: maximum 10 mEq/L per day (≈0.5 mEq/L/hr) to avoid cerebral oedema from rapid water entry into brain cells.
IV fluid choice: 5% dextrose (D5W) provides free water once glucose metabolised; 0.45% NaCl if patient also has sodium deficit.

References

Adrogue HJ, Madias NE. Hypernatremia · N Engl J Med (2000)

Sterns RH. Disorders of plasma sodium — causes, consequences, and correction · N Engl J Med (2015)

For educational purposes only. Not for clinical decision-making without professional oversight.