Sodium Deficit In Hyponatremia Calculator

Sodium Deficit In Hyponatremia Calculator for Endocrinology. The output is a sodium quantity (mmol) and the corresponding saline volume to reach the entered target. It should be paired with strict rate limits, not a single endpoint: correct by no more than 8 mmol/L in any 24 hours (and no more than 4-6 mmol/L in 24 h in high-risk patients: alcoholism, malnutrition, hypokalemia, advanced liver disease, or Na below 105 mmol/L). Because these formulas assume a closed system and ignore ongoing urinary losses, the measured rise routinely exceeds prediction, so the calculated deficit is a ceiling to titrate against, not a fixed dose. Overshoot mandates active relowering with dextrose 5% water and desmopressin to prevent osmotic demyelination.

How this calculator works

This tool estimates the total-body sodium deficit needed to raise a hyponatremic patient's serum sodium to a target value, using the classic formula: Na deficit (mmol) = total body water (TBW) x (target Na - measured Na), where TBW = weight (kg) x a distribution factor (0.6 for men/children, 0.5 for women and elderly men, ~0.45 for elderly women). Some implementations instead apply the Adrogue-Madias equation, which predicts the change in serum Na from 1 L of a chosen infusate: change in Na = (infusate Na - serum Na) / (TBW + 1). The deficit figure converts directly into a volume of 3% saline (513 mmol Na/L) or normal saline (154 mmol/L), and the chosen correction rate then sets the infusion rate.

When to use this calculator

Use for acute planning of hypertonic or isotonic saline therapy in hypovolemic and euvolemic (SIADH) hyponatremia, especially when serum Na is below 120 mmol/L or the patient has seizures, obtundation, or vomiting. It is most useful as a starting estimate for the first several hours of correction. Do NOT use it as a substitute for frequent (every 2-4 h) serum sodium remeasurement, and it is unreliable in hyperglycemic pseudohyponatremia, ongoing free-water or solute diuresis, and beer potomania/low-solute states where correction can overshoot dramatically.

Inputs used

  • Measured sodium
  • Target sodium
  • Weight
  • Sex or total body water estimate

Clinical interpretation

The output is a sodium quantity (mmol) and the corresponding saline volume to reach the entered target. It should be paired with strict rate limits, not a single endpoint: correct by no more than 8 mmol/L in any 24 hours (and no more than 4-6 mmol/L in 24 h in high-risk patients: alcoholism, malnutrition, hypokalemia, advanced liver disease, or Na below 105 mmol/L). Because these formulas assume a closed system and ignore ongoing urinary losses, the measured rise routinely exceeds prediction, so the calculated deficit is a ceiling to titrate against, not a fixed dose. Overshoot mandates active relowering with dextrose 5% water and desmopressin to prevent osmotic demyelination.

Worked example

A 70 kg man with serum Na 110 mmol/L and seizures. TBW = 70 x 0.6 = 42 L. To reach a first-day target of 118 mmol/L (an 8 mmol/L rise), deficit = 42 x (118 - 110) = 336 mmol. Delivered as 3% saline (513 mmol/L), that is about 655 mL. To keep the 24-hour rise at roughly 6-8 mmol/L, this volume is spread over the day (about 27 mL/h), with an initial 100-150 mL bolus given for the active seizure, then serum Na rechecked at 2-3 hours.

Limitations and safety notes

The formulas treat the body as a static container and ignore renal water handling; when the stimulus to ADH is removed (volume repletion, cortisol replacement, stopping a drug), a water diuresis can cause the sodium to rise far faster than any equation predicts. They systematically under-predict the rise in hypovolemic hyponatremia and beer potomania, where autocorrection is common. They do not apply to hyperglycemia (correct the measured Na for glucose first) or to patients with significant ongoing GI, renal, or third-space losses. In critically ill children the Adrogue-Madias variant has only small-cohort prospective support.

Frequently asked questions

Why did my patient's sodium rise faster than the calculator predicted?

The formulas assume no ongoing urine output. When ADH is suppressed (after volume repletion, glucocorticoid replacement, or drug withdrawal), the kidney excretes dilute urine and free water is lost, so serum Na climbs beyond the model. This is why every protocol requires serial sodium measurement rather than trusting the initial calculation.

Should I use the sodium-deficit formula or the Adrogue-Madias formula?

They answer slightly different questions. The deficit formula gives a total mmol needed to reach a target and is convenient for planning a full day's dose; Adrogue-Madias predicts the Na change per liter of a specific infusate and is better for choosing between fluids. Both share the same TBW assumption and both under-predict when a water diuresis begins.

What sodium correction rate should I target?

For most patients, aim for a rise of 6-8 mmol/L over 24 hours; the hard safety limit is 8 mmol/L per 24 h, tightened to 4-6 mmol/L in high-risk patients (chronic hyponatremia, alcoholism, malnutrition, hypokalemia, liver disease, or serum Na below 105 mmol/L). In symptomatic acute hyponatremia, a rapid 4-6 mmol/L bump with a 100-150 mL bolus of 3% saline relieves cerebral edema; the daily ceiling still applies.

How do I handle overcorrection?

If the 24-hour rise approaches or exceeds the limit, stop hypertonic saline, give free water as 5% dextrose, and administer desmopressin to halt the water diuresis. Proactive DDAVP clamping is increasingly used from the outset in very-low-sodium patients to make the rise predictable.

References

  • Adrogué HJ, Madias NE. Hyponatremia. N Engl J Med. 2000;342(21):1581-9. PMID: 10824078.
  • Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1-47. PMID: 24569125.
  • Assadi F, Azarfar A, Bazargani B, et al. Validity of the Adrogué-Madias Formula for the Management of Acute Dysnatremias in Critically Ill Children: A Prospective Multicenter Analysis. Pediatr Emerg Care. 2023;39(9):707-714. PMID: 37167202.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: June 8, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

Related reviewed calculators