Metabolism
Anion gap calculator
Calculate the serum anion gap from sodium, chloride, and bicarbonate, with an optional albumin correction, and see how the result is used to work up a metabolic acidosis.
Enter your values above to see your result and what it means.
What the gap actually represents
Blood is electrically neutral, so the positive and negative charges in it must balance. A routine panel measures one major cation, sodium, and two major anions, chloride and bicarbonate. Subtracting the measured anions from the measured cation leaves a residue that stands for everything not measured, principally albumin along with phosphate, sulphate, and organic acids. That residue is the anion gap, and it rises when an unmeasured acid accumulates.
Why albumin correction is not optional in practice
Albumin is the dominant unmeasured anion, so hypoalbuminaemia lowers the baseline gap and can hide a genuine acidosis. The gap falls by roughly 2.5 mmol/L for every 1 g/dL that albumin drops below 4.0 g/dL. A critically ill patient with an albumin of 2.0 g/dL and an apparently normal gap of 11 has a corrected gap of about 16, which changes the interpretation entirely. Supplying albumin above adds this corrected figure alongside the raw one.
Potassium in the formula
Some sources calculate the gap as sodium plus potassium, minus chloride and bicarbonate. Including potassium raises the result by roughly 4 mmol/L and shifts the reference range up accordingly. This calculator uses the more common sodium-only form. The two are both valid provided the reference range matches the formula, and mixing one formula with the other range is the error to avoid.
Frequently asked questions
What is a normal anion gap?
On the ion-selective electrodes used by modern laboratories, roughly 8 to 12 mmol/L using the sodium-only formula. Older analytical methods produced higher normals of around 12 to 16, which is why some references still quote that range. Use the range your own laboratory publishes.
What causes a high anion gap?
Accumulation of unmeasured acid anions. In practice that means lactic acidosis, ketoacidosis from diabetes, alcohol, or starvation, advanced kidney failure, and ingestion of methanol, ethylene glycol, or salicylates.
Why does a low albumin matter?
Albumin is the largest unmeasured anion, so a low albumin lowers the baseline gap. That can mask a real high anion gap acidosis, which is why the corrected gap, roughly 2.5 mmol/L added per 1 g/dL of albumin below 4.0, is the more informative figure in ill patients.
Should I include potassium?
Either form is acceptable as long as the reference range matches it. This calculator omits potassium, in line with the more widely used convention, and the 8 to 12 mmol/L range quoted here corresponds to that formula.