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Anion Gap Calculator with Albumin Correction

This anion gap calculator works out the serum anion gap from sodium, chloride and bicarbonate – with or without potassium – and corrects it for low albumin using the Figge formula. Example: Na 140, Cl 104 and HCO₃ 24 mmol/L give an anion gap of 12 mmol/L; with an albumin of 3.0 g/dL the corrected value is 14.5 mmol/L.

Source: Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol 2007;2(1):162–174 (definition: sodium minus the sum of chloride and bicarbonate). Updated: .

mmol/L
mmol/L and mEq/L are the same number for these ions.
mmol/L
mmol/L
On many lab reports this appears as total CO₂.
g/dL
Leave empty to skip the albumin correction.
mmol/L
Reference ranges differ between laboratories – use the one printed on your report.
My ToolboxYour inputs are saved in this browser only.

Result

Result
Anion gap = 12 mmol/L · albumin-corrected 14.5 mmol/L (above the entered upper limit of 12 mmol/L)
Your values
MeasureValue
Anion gap (without potassium)12 mmol/L
Albumin correction+2.5 mmol/L
Albumin-corrected anion gap14.5 mmol/L
Upper reference limit entered12 mmol/L
Compared with the limitabove the entered upper limit of 12 mmol/L
Working
  • Anion gap = Na − (Cl + HCO₃) = 140 − (104 + 24) = 12 mmol/L.
  • Albumin correction (Figge): 12 + 2.5 × (4 − 3) = 14.5 mmol/L (albumin in g/dL).
Reported reference ranges (without potassium)
Measurement methodReported range
Older methods (flame photometry, 1970s)8–16 / 9–17 mmol/L
Laboratories using ion-selective electrodes3–11 / 5–12 mmol/L
Upper limit many laboratories still list15–16 mmol/L
A calculated value for orientation – it does not replace a medical assessment. Whether an anion gap is raised depends on the reference range of the laboratory that measured the values; the calculator compares only with the limit you enter and makes no diagnosis. Acid-base disorders are assessed together with blood gas results and the clinical picture.

How it is calculated

The anion gap is the difference between the measured positive ions (sodium, sometimes potassium) and the measured negative ions (chloride and bicarbonate) in serum. Blood is electrically neutral, so the “gap” is made up of anions that routine tests do not measure – mainly albumin, plus phosphate, sulphate and organic acids. Doctors use it above all to sort the causes of a metabolic acidosis.

Formula

Without potassium (the usual form): anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). With potassium: anion gap = Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻). All values are in mmol/L; for these singly charged ions that is the same number as mEq/L. Potassium varies little, which is why most clinicians leave it out; with it, the result is higher by the potassium value, about 4 mmol/L.

Worked example

Na 140, Cl 104, HCO₃ 24 mmol/L: anion gap = 140 − (104 + 24) = 12 mmol/L. With a potassium of 4.0 mmol/L the second formula gives 140 + 4 − 128 = 16 mmol/L. A sample with Na 135, Cl 100 and HCO₃ 10 mmol/L has an anion gap of 135 − 110 = 25 mmol/L.

Albumin correction (Figge)

Albumin carries a large share of the unmeasured negative charge. If albumin is low, the anion gap falls and can look normal although extra acids are present. Figge and colleagues therefore proposed: corrected anion gap = anion gap + 0.25 × (normal albumin − measured albumin) with albumin in g/L, or a factor of 2.5 with albumin in g/dL. In the example, an albumin of 3.0 g/dL and a normal value of 4.0 g/dL give 12 + 2.5 × 1.0 = 14.5 mmol/L. Which “normal” albumin is used varies – 4.0 and 4.4 g/dL are both common – so you can choose.

What is a normal anion gap?

There is no single number. With the older methods of the 1970s the range was 8–16 or 9–17 mmol/L. Laboratories using ion-selective electrodes have reported lower ranges of 3–11 or 5–12 mmol/L, yet many laboratories still list an upper limit of 15–16 mmol/L. That is why this calculator asks for the upper limit on your own report instead of applying a fixed threshold.

Frequently asked questions

What is a normal anion gap?

It depends on the laboratory method. Reported reference ranges without potassium run from 3–11 or 5–12 mmol/L (ion-selective electrodes) to 8–16 mmol/L (older methods); many laboratories list an upper limit of 15–16 mmol/L. Use the range printed on your own report.

Should potassium be included?

Most clinicians leave it out, because potassium varies little. If you include it, the result is higher by the potassium value – 16 instead of 12 mmol/L in the example with a potassium of 4 – and the reference range has to be the one for that formula.

How do I correct the anion gap for albumin?

Add 2.5 mmol/L for every 1 g/dL that albumin lies below normal (0.25 per g/L). With an anion gap of 12, an albumin of 3.0 g/dL and a normal value of 4.0 g/dL, the corrected anion gap is 14.5 mmol/L.

Are mmol/L and mEq/L the same here?

Yes. Sodium, potassium, chloride and bicarbonate each carry a single charge, so 1 mmol/L equals 1 mEq/L and the anion gap is the same number in both units.

What does a high anion gap mean?

It suggests that unmeasured anions have accumulated, most commonly in a metabolic acidosis caused by lactate, ketones or certain poisonings. A single calculated value is not a diagnosis; it has to be interpreted by a doctor together with blood gases and symptoms.

Sources and legal basis

As of: · Editorial policy

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