How is the sodium deficit calculated?

The deficit equals total body water multiplied by the difference between the target and current sodium. Total body water is the weight in kilograms times a factor of about 0.6 in men, 0.5 in women, and lower in elderly patients. The result is in millimoles of sodium.

How is the fluid volume worked out?

Each fluid carries a known sodium concentration: 3% saline has 513 mmol per litre and 0.9% saline has 154 mmol per litre. Dividing the sodium deficit by that concentration gives the litres of fluid needed, which the tool converts to millilitres and an hourly rate.

Why is the correction rate limited?

Raising sodium too quickly can cause osmotic demyelination syndrome, a severe and often irreversible brain injury. The accepted limit is no more than 8 mmol per litre in any 24-hour period. The tool flags a plan whose projected 24-hour rise would breach this ceiling.

What interim target should I aim for?

In severe symptomatic hyponatraemia the usual aim is a prompt rise of 4 to 6 mmol per litre to relieve cerebral oedema, then to stop and observe rather than chase a normal value. The remaining correction is then made slowly over the following days.

Is this formula exact?

No. The Adrogué–Madias style deficit ignores ongoing urinary and insensible losses and rapid free-water shifts, which is why sodium can rise faster than predicted. Always recheck the level every two to four hours and adjust, and use the calculator only under senior supervision.

Sodium Deficit Calculator for Hyponatraemia Replacement

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Correcting low serum sodium is a balance between relieving dangerous cerebral oedema and avoiding the catastrophe of over-rapid correction. This calculator works out the sodium deficit and the volume of saline needed to reach a chosen interim target, while watching the safe correction ceiling.

How it works

The deficit is the amount of sodium that must be added to the body water to lift the concentration to your target:

TBW (L)        = weight_kg x factor   (0.6 M, 0.5 F, lower if elderly)
deficit (mmol) = TBW x (target_Na - current_Na)
volume (L)     = deficit / fluid_sodium_concentration
                 3% NaCl = 513 mmol/L, 0.9% NaCl = 154 mmol/L

Dividing the volume by the planned infusion time gives the rate in millilitres per hour. The tool also projects the implied 24-hour rise from your chosen rate and warns if it would exceed the 8 mmol/L ceiling that guards against osmotic demyelination.

Why the correction ceiling matters

Osmotic demyelination syndrome (ODS) is the clinical catastrophe of over-rapid correction. It arises when brain cells, which have adapted to a chronically low-sodium environment by releasing organic osmolytes, are suddenly exposed to a rapidly rising extracellular tonicity they cannot match. The consequence — patchy myelin destruction, classically in the pons — can produce a locked-in-like state, dysarthria, quadriparesis, or death. It is largely irreversible.

The 8 mmol/L per 24-hour ceiling is the widely cited guard against this risk. Some guidelines quote a tighter limit of 6–8 mmol/L in patients at highest risk (severe malnutrition, alcoholism, liver disease), where the brain’s adaptation is most pronounced. This tool shows the warning at 8 mmol/L; for high-risk patients, consider using a lower ceiling when reading the output.

Worked example and cautions

A 70 kg man with a sodium of 118, targeted to 123 over 6 hours, has a total body water of 42 L and a deficit of 42 × 5 = 210 mmol. Delivered as 3% NaCl, that is 210 ÷ 513 ≈ 0.41 L (410 mL), roughly 68 mL per hour. The planned rise of 5 mmol/L over 6 hours extrapolates to a 24-hour projection well within the ceiling, so the tool confirms the plan is safe. The interim target of 123 is appropriate — the aim is to relieve symptoms, not to normalise the sodium in one step.

Why the formula underestimates real rises

The Adrogué–Madias framework treats total body water as a closed compartment, but it is not. SIADH patients are actively retaining water, and if the stimulus (pain, nausea, medications) resolves during treatment the sodium can rise faster than predicted. Similarly, a patient given hypertonic saline who then starts to pass large volumes of dilute urine will see an accelerated rise. This “auto-correction” phenomenon is why 2–4 hourly sodium checks are essential during correction — the formula gives a starting infusion rate, not a guarantee of what will happen.

Treat the output as a first approximation, recheck the level every two to four hours, and slow or stop infusion once the target rise for that phase is reached. Manage severe hyponatraemia only with senior input and, where available, alongside an endocrinology or critical-care opinion.