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
This formula is intended for hypernatraemia (Na >145 mEq/L)
TBW fraction: male 60%, female 50%, elderly female 45%
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)
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