Corrected Sodium Calculator
Estimate what the serum sodium would be without the dilutional effect of hyperglycaemia — the calculation used at the bedside in diabetic ketoacidosis and hyperosmolar states.
Measured sodium: — mmol/L
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Why Glucose Changes the Measured Sodium
Glucose is osmotically active but largely confined to the extracellular fluid. When it rises, water moves out of the cells into the extracellular space, diluting the sodium that is already there. The laboratory reports a sodium that is genuinely low in concentration but not a reflection of total body sodium. The corrected sodium estimates the concentration that would be measured if the glucose were normal, which is why it changes the interpretation so often in diabetic ketoacidosis and hyperosmolar hyperglycaemic states.
The Two Formulas
- Katz (1973): corrected Na = measured Na + 1.6 × [(glucose mg/dL − 100) ÷ 100]
- Hillier (1999): corrected Na = measured Na + 2.4 × [(glucose mg/dL − 100) ÷ 100]
- In SI units the same relationships apply per 5.5 mmol/L of glucose above 5.5 mmol/L
Katz was derived at moderate glucose levels, where the correction is roughly 1.6 mmol/L per 100 mg/dL. Hillier re-examined the relationship in patients with glucose above 400 mg/dL and found the true coefficient closer to 2.4, so Katz underestimates the correction at very high glucose. This calculator shows both so the difference is visible rather than hidden: for a sodium of 130 with a glucose of 450 mg/dL, Katz gives 135.6 and Hillier gives 138.4 — one value sits at the lower limit of normal, the other comfortably inside it.
How It Is Used
- Deciding on fluid composition in DKA and HHS. A high corrected sodium argues against large volumes of isotonic saline; a low one supports them.
- Tracking the response to treatment. As glucose falls, the corrected sodium should fall with it, and the measured sodium should rise towards it. If the measured sodium keeps falling while the corrected value is stable, free water is being given faster than it is being lost.
- Recognising a mixed picture. A corrected sodium that remains below 135 after treatment suggests a second cause — diuretics, SIADH, adrenal insufficiency, or ongoing gastrointestinal losses.
- Interpreting the osmolality. Calculated osmolality and the corrected sodium together separate dilutional from depletional hyponatraemia.
Limitations
- Both formulas are population regressions: individual patients correct differently, particularly with coexisting renal failure or acidosis.
- They assume glucose is the only osmotically active solute that has changed; mannitol, glycine, ethanol and renal failure all break that assumption.
- They describe concentration, not total body sodium — volume status still has to be assessed clinically.
- Rapid changes in glucose make any single corrected value a snapshot rather than a target.
Frequently Asked Questions
Which formula should I use?
Either is defensible at ordinary glucose levels. Above roughly 400 mg/dL the Hillier coefficient matches the observed shift better, and showing both avoids false precision.
Does a corrected sodium of 140 mean the patient is fine?
It means the sodium concentration is explained by the glucose. Total body sodium and volume status can still be depleted, which is why the corrected value is read alongside blood pressure, urine output and clinical assessment.
How fast should the corrected sodium change?
It should track the fall in glucose. A corrected sodium that rises while glucose falls suggests the patient is losing free water; one that falls suggests over-replacement with hypotonic fluid. Both need clinical attention.