Estimate the serum sodium change from 1 litre of fluid with the Adrogue-Madias formula: (infusate Na+K − serum Na) / (TBW + 1).
The Adrogue-Madias formula predicts how serum sodium will move after infusing 1 litre of a chosen fluid. It helps you pick the fluid and infusion rate in both hyponatraemia and hypernatraemia. Adrogue and Madias published it in NEJM in 2000: deltaNa = (infusate Na + infusate K − serum Na) / (TBW + 1), where TBW (total body water) = weight × factor 0.6 in men, 0.5 in women, 0.45 in elderly patients. The sign shows direction: hypertonic fluids (3% and 5% NaCl) raise sodium, hypotonic fluids (0.45% NaCl, 5% dextrose) lower it, and normal saline sits in between. In hyponatraemia raise sodium by no more than 8–10 mmol/L per 24 h to avoid osmotic demyelination; in hypernatraemia lower sodium by no more than 10 mmol/L per 24 h. The calculator estimates how many litres of the fluid over a day stay within that limit. The correction limits come from the European hyponatraemia guideline (Spasovski, 2014), which SEMI and SEEN follow in Spain. This is educational reference information, not a substitute for bedside calculation and serum sodium checks every 2–6 hours.
Adrogue HJ, Madias NE. N Engl J Med 2000;342(21):1581-1589
Formula version: adrogue-madias-2000-v1