Clinical & Hospital Pharmacy
Anion Gap Calculator
Calculates the serum anion gap from sodium, chloride and bicarbonate, with an optional albumin correction.
Anion gap
12.0mEq/L
Normal anion gap
Electrolyte values
From the patient's basic metabolic panel (serum).
Normal 135–145 mEq/L.
Normal 96–106 mEq/L.
Normal 22–28 mEq/L. May be reported as total CO₂.
Try an example
Examples fill albumin too; switch to Albumin-corrected to use it.
Working
The numbers you entered, plugged into the formula.
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
Albumin-corrected AG = AG + 2.5 × (4 − albumin in g/dL)
Na⁺ is sodium, Cl⁻ chloride and HCO₃⁻ bicarbonate, all in mEq/L. Albumin is the largest unmeasured anion, so every 1 g/dL it falls below 4 g/dL lowers the gap by about 2.5 mEq/L. The correction adds that back, unmasking a raised gap in a patient with low albumin.
Here a gap above 12 mEq/L is read as elevated and below 6 mEq/L as low. When the correction is on, the interpretation is based on the corrected value.
Common questions
Albumin carries negative charge and makes up most of the normal anion gap. In a patient with low albumin (liver disease, nephrotic syndrome, critical illness) the gap is artificially low, so an acid that would normally raise it can leave it looking normal. The correction adds 2.5 mEq/L for every 1 g/dL of albumin below 4.
Some references use Na + K − (Cl + HCO₃), which gives a normal range about 4 mEq/L higher. This calculator uses the more common form without potassium — compare your result against a range for the same formula.
It is uncommon and most often reflects low albumin or a lab error. Other causes include paraproteins (multiple myeloma), lithium or bromide toxicity, and severe hypercalcaemia or hypermagnesaemia.
ΔAG / ΔHCO₃ — how much the gap has risen compared with how much bicarbonate has fallen. In a pure high-gap acidosis the two move roughly together; a ratio well below or above that suggests a mixed disorder (an additional normal-gap acidosis or a metabolic alkalosis).
Yes. Most chemistry analysers report total CO₂, which is mostly bicarbonate and is what the anion gap is normally calculated from.
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