Anion Gap Calculator

mmol/L
mmol/L
mmol/L
mmol/L

Optional. Including it raises the gap and the reference range with it.

g/dL

Optional. A low albumin hides a raised gap until it is corrected.

Anion gap

– mmol/L

Enter sodium, chloride and bicarbonate.

Reference range
–
Corrected for albumin
–

The working

      Albumin correction: Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807-10.

      This works out the arithmetic. The order, the product in your hand and local policy still govern what is given.

      The anion gap formula

      anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

      Serum is electrically neutral, so the positive and negative charges in it balance exactly. A laboratory panel measures only some of each, and the gap between the measured cations and the measured anions is the space taken up by everything not on the panel: albumin, phosphate, sulphate, lactate and any organic acid that happens to be present.

      That is why a raised gap is informative. It says unmeasured anions have accumulated, without saying which, and the differential for that is short enough to be useful.

      Reference ranges and the potassium convention

      Two versions of the formula are in routine use and they have different normal ranges. Without potassium the usual range is 8 to 12 mmol/L; with it, 12 to 16. Both are correct; mixing one formula with the other's range is not, and it is the commonest way this calculation is misread.

      PictureNa⁺Cl⁻HCO₃⁻Gap
      Normal1401042412
      Raised gap1401001228
      Normal gap acidosis1401121612
      Low albumin, corrected1401042412 (17 corrected)

      Why a low albumin hides a raised gap

      corrected gap = gap + 2.5 × (4.0 − albumin in g/dL)

      Albumin is negatively charged at physiological pH and accounts for a large share of the normal gap. When albumin falls, the measured gap falls with it, and a patient with a genuine accumulation of organic acid can return a gap that looks unremarkable.

      The last row of the table above is the case that matters: a gap of 12 reads as normal until the albumin of 2.0 g/dL is accounted for, at which point it is 17. The same reasoning drives thecorrected calcium calculation, where albumin binding rather than albumin charge is what shifts the number.

      Questions about the anion gap

      What is the anion gap formula?
      Sodium minus the sum of chloride and bicarbonate, in mmol/L. Some units include potassium, in which case the formula is sodium plus potassium, minus chloride plus bicarbonate.
      What is a normal anion gap?
      Without potassium, roughly 8 to 12 mmol/L. With potassium included, roughly 12 to 16 mmol/L. Modern analysers measure chloride slightly differently from older ones, so many laboratories now quote a lower range, and the range on your own report is the one to use.
      Why does albumin need correcting for?
      Albumin is itself an unmeasured anion, and a large part of the normal gap. When albumin is low the apparent gap falls, which can mask a genuine high-anion-gap acidosis. Adding 2.5 mmol/L for every 1 g/dL the albumin sits below 4.0 restores it.
      Should potassium be included?
      Either convention is acceptable as long as the reference range matches. Potassium is a small and relatively stable contributor, so most units leave it out. What matters is not mixing a gap calculated one way with a range quoted for the other.