Sodium Correction Rate Calculator (Adrogue-Madias)
Predict the change in serum sodium per litre of infusate with the Adrogue-Madias formula — 3% saline, 0.9% saline, 0.45% saline, D5W — infusion rate to hit a safe hourly and 24-hour target in hyponatraemia or hypernatraemia.
How it works
ΔNa per litre = (Na infusate + K infusate − Na serum) ÷ (TBW + 1); TBW = weight × fraction (0.6 men, 0.5 women/elderly men, 0.45 elderly women)
The Adrogue-Madias equation estimates how far one litre of a given fluid will move the serum sodium, treating the body as a single compartment of total body water plus the litre added. Hypertonic 3% saline raises sodium by roughly 10 mmol/L per litre in an average adult, which is why it is given in small 100–150 mL boluses for symptomatic hyponatraemia; free water (D5W) lowers it by about 3 mmol/L per litre. Potassium in the bag counts like sodium because it enters cells and pushes sodium out. The formula ignores ongoing urine and insensible losses, so it tends to under-predict the correction when the patient is excreting dilute urine (for example after the cause of SIADH resolves) — which is exactly when overcorrection happens. Measure sodium every two to four hours and treat the calculation as a starting rate.
Worked example
A 65 kg woman (TBW 32.5 L) with sodium 118 on 3% saline: (513 − 118) / 33.5 = +11.8 mmol/L per litre. To raise sodium 6 mmol/L over 6 hours: 509 mL, about 85 mL/h.
Frequently asked questions
What is the maximum safe rise in chronic hyponatraemia?
Most guidelines: 4–6 mmol/L in the first 6 hours to relieve symptoms, then no more than 8 mmol/L in any 24 hours if risk factors for osmotic demyelination are present (alcoholism, malnutrition, liver disease, very low sodium), otherwise 10–12.
Why does 0.9% saline sometimes worsen hyponatraemia?
In SIADH the kidney excretes the sodium load in concentrated urine but retains some water, so isotonic saline can lower sodium further — the formula does not capture this; use hypertonic saline or fluid restriction.
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Clinical decision tool for education and for use by trained clinicians alongside judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.