Why sodium must be corrected carefully
Hyponatremia, a serum sodium below 135 mEq/L, is the most common electrolyte disorder in hospital. Severe or rapidly developing hyponatremia can cause brain swelling, seizures and coma. But in chronic hyponatremia, brain cells have adapted to the low sodium, and raising it too fast can cause osmotic demyelination syndrome, an often permanent injury. Doctors therefore need to predict how much a given fluid will change the sodium.
The Adrogué–Madias formula
Published in the New England Journal of Medicine in 2000 by Horacio Adrogué and Nicolaos Madias:
Change in serum Na per liter = (infusate Na + infusate K − serum Na) ÷ (total body water + 1)
Total body water is estimated from weight: 60% for children and adult men, 50% for adult women and older men, 45% for older women. The “+1” accounts for the liter of fluid being added.
Common infusates:
- 3% saline: 513 mEq/L of sodium
- 0.9% saline: 154 mEq/L
- Lactated Ringer’s: 130 mEq/L
- 0.45% saline: 77 mEq/L
- 5% dextrose in water: 0 mEq/L, which lowers the sodium
Worked example
A 132 lb (59.9 kg) adult woman has a serum sodium of 120 mEq/L. The team plans to raise it by 8 mEq/L over 24 hours with 3% saline:
- Total body water: 59.9 × 0.5 = 29.9 L.
- Change per liter: (513 − 120) ÷ (29.9 + 1) = +12.7 mEq/L.
- Volume for +8: 8 ÷ 12.7 = 630 mL, or about 26 mL/h over 24 hours.
Correction limits
For chronic hyponatremia, most guidelines recommend raising sodium by no more than 8 to 10 mEq/L in the first 24 hours, and no more than 8 in the following 24 hours for patients at high risk of osmotic demyelination: alcohol use disorder, malnutrition, low potassium or liver disease. A rise above 10 is shown as a danger flag. Patients with severe symptoms such as seizures need faster initial treatment, typically small boluses of hypertonic saline, under close monitoring.
Why the prediction can be wrong
The formula assumes the only change is the fluid given. In reality, patients keep drinking, eating and producing urine. Once the cause is fixed, such as low blood volume, a thiazide diuretic or a drug causing SIADH, the kidneys can suddenly excrete large volumes of dilute urine and the sodium can overshoot. Adding potassium chloride also raises the sodium, which is why potassium is included in the formula.
Sodium should be rechecked every two to four hours at the start of treatment, with urine output monitored. If the correction goes too fast, doctors can slow it or even lower the sodium again with free water and desmopressin.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.