Sodium Deficit (Hyponatraemia)

Background

The sodium deficit estimates how much sodium is needed to raise a patient's serum sodium to a chosen target in hyponatraemia, providing a starting point for planning correction. It is based on total body water, calculated as a sex-specific fraction of body weight (about 0.6 in men and 0.5 in women, lower in the elderly), multiplied by the desired rise in sodium. The formula gives an estimate of the millimoles of sodium required, not a prescription. It is one input into a wider, carefully monitored correction plan.

Interpreting the result

A larger estimated deficit reflects either a greater intended rise in sodium or a larger total body water, and translates into a slower, more cautious approach. The key safety constraint is not the deficit itself but the rate of correction: serum sodium should generally rise by no more than about 8–10 mmol/L in 24 hours to avoid osmotic demyelination syndrome, with even tighter limits in high-risk patients. The estimate is therefore used to plan the direction and approximate magnitude of correction, with frequent re-measurement guiding the actual pace.

Worked example

For a 70 kg man (total body water ≈ 0.6 × 70 = 42 L) whose sodium is 120 mmol/L and target is 128 mmol/L, the deficit ≈ 42 × (128 − 120) = 336 mmol of sodium to achieve an 8 mmol/L rise over the planned period.

Pearls / pitfalls

• The formula estimates a quantity, but the safety-critical number is the rate of rise — overly rapid correction risks osmotic demyelination syndrome.
• Ongoing renal water and electrolyte losses are not captured, so the actual response often differs from the estimate; re-measure sodium frequently.
• High-risk patients (severe or chronic hyponatraemia, alcohol use, malnutrition, hypokalaemia) need even more cautious limits.
• Acute symptomatic hyponatraemia and chronic hyponatraemia are managed differently; the deficit alone does not determine urgency.

Evidence & validation

The deficit derives from total-body-water physiology described in standard texts and reviews such as Adrogué and Madias; it is an estimating tool, and guidelines (including European hyponatraemia guidance) emphasise correction limits and frequent monitoring over reliance on any single formula.

Frequently asked questions

References

1. Adrogué HJ, Madias NE. Hyponatremia. N Engl J Med. 2000;342(21):1581–1589.
2. Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1–G47.

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