What it is
In hyponatremia the sodium deficit estimates the total milliequivalents of sodium needed to raise serum Na from its measured value to a chosen target. It quantifies the gap; it does not set the infusion rate — the rate is bounded by a daily correction ceiling to prevent osmotic demyelination.
Formula
$$ TBW = fraction \times weight $$ $$ Deficit\ (mEq) = (Na_{target} - Na_{measured}) \times TBW $$
The total body water fraction defaults to 0.6 (children / older infants); use 0.7 for infants under 1 year. A conservative initial target is often 125 mEq/L rather than full normalisation in one step.
When to use
Chronic or asymptomatic hyponatremia, to plan total sodium replacement toward a safe target. Acute symptomatic hyponatremia (seizures, obtundation) is an emergency managed by hypertonic saline boluses — not by this deficit calculation.
Worked example
A 20 kg child, sodium 120 mEq/L, target 125 mEq/L, fraction 0.6.
$$ TBW = 0.6 \times 20 = 12\ \text{L} $$ $$ Deficit = (125 - 120) \times 12 = 60\ \text{mEq} $$
So ~60 mEq of sodium to reach 125 mEq/L — delivered slowly, respecting the daily ceiling, not as a bolus.
Safety: the correction-rate ceiling
Over-rapid correction of chronic hyponatremia causes osmotic demyelination syndrome (central pontine myelinolysis). The limits this tool enforces in its note:
- Do not raise serum Na more than ~8 mEq/L per 24 hours.
- Use a tighter ≤6 mEq/L per 24h in high-risk children — chronic, malnourished, severe (Na ≤105), hypokalemic, or liver disease.
- Recheck Na frequently; halt at the daily limit even if still below target.
Acute symptomatic seizures are managed separately. Give 3% saline 2 mL/kg (may repeat, ~max 4–6 mL/kg) to raise Na just enough to stop the seizure — typically a rise of ~4–6 mEq/L — not by infusing the calculated deficit.
Pitfalls
- The deficit is a total, not a rate. It does not tell you how fast to give it; the 24h ceiling does.
- Beware autocorrection. Once the stimulus (e.g. desmopressin, volume depletion) resolves, a brisk aquaresis can overshoot — monitor and be ready to re-administer free water or DDAVP.
- Don’t aim for normal Na in 24h in chronic hyponatremia; partial correction is the goal.
- Hypokalemia correction itself raises serum Na — count it toward the daily total.
Run it: Sodium Deficit (Hyponatraemia)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.