Nephrology
Free Water Deficit
Estimated water deficit in hypernatraemia.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The free water deficit estimates how much electrolyte-free water a patient with hypernatraemia has lost, to guide replacement. It is calculated as total body water × ((current sodium / target sodium) − 1), where total body water is estimated as a fraction of body weight (about 0.6 in men and 0.5 in women, with lower fractions in older and female patients). The deficit reflects the water needed to dilute the elevated sodium back toward normal. It provides only a starting estimate and assumes the target sodium is the desired endpoint.
Interpreting the result
A larger deficit indicates more severe water loss and a greater volume of free water required for correction. The calculated figure guides the rate and volume of replacement but must be applied cautiously: sodium should be lowered slowly to avoid cerebral oedema. The estimate does not account for ongoing losses (urine, insensible, gastrointestinal), which must be added separately, and frequent re-measurement of sodium is essential to titrate therapy rather than relying on the initial number.
Worked example
An 80 kg man with a sodium of 160 mmol/L, aiming for 140 mmol/L: total body water ≈ 0.6 × 80 = 48 L; deficit = 48 × ((160 / 140) − 1) ≈ 48 × 0.143 ≈ 6.9 L. This is the free water deficit, to be replaced gradually alongside ongoing losses while sodium is monitored.
Critical actions
Correct hypernatraemia slowly (aim to lower sodium by ≤10–12 mmol/L per 24h) to avoid cerebral oedema. This is an estimate; reassess and recalculate frequently.
Pearls / pitfalls
- Correct chronic hypernatraemia slowly — generally aiming to lower sodium by no more than about 10–12 mmol/L per 24 hours — to avoid cerebral oedema.
- The formula estimates the existing deficit only; ongoing urinary and insensible losses must be replaced in addition.
- Total body water fractions are approximations and fall with age and in women, so the estimate is inherently imprecise.
- It is a starting point, not a prescription — recheck sodium frequently and recalculate as values change.
Evidence & validation
The approach is based on classic body-water physiology summarised by Adrogué and Madias in their widely cited review of hypernatraemia, and underpins standard guidance that hypernatraemia should be corrected slowly to prevent cerebral oedema.
Frequently asked questions
How quickly should hypernatraemia be corrected?
Chronic hypernatraemia should be corrected gradually, typically aiming to lower the sodium by no more than about 10–12 mmol/L over 24 hours. Correcting too fast risks cerebral oedema, so frequent sodium monitoring is essential.
Does the free water deficit include ongoing losses?
No. The calculation estimates only the water already lost. Continuing urinary, gastrointestinal and insensible losses must be replaced in addition to the calculated deficit.
Why does sex change the result?
Total body water is estimated as a fraction of body weight, and this fraction is lower in women (about 0.5) than men (about 0.6), and lower again in older people. A smaller fraction gives a smaller estimated deficit.
Is the calculated deficit exact?
No, it is an approximation based on assumed body-water fractions and a target sodium. It should be used to guide initial therapy, with repeated sodium measurements used to adjust the actual replacement.
References
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493–1499.
- Sterns RH. Disorders of plasma sodium — causes, consequences, and correction. N Engl J Med. 2015;372(1):55–65.
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