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Anion Gap Calculator

Calculate the serum anion gap from sodium, chloride and bicarbonate, add potassium if your laboratory does, correct it for a low albumin, and see the delta ratio, with every step written out. For healthcare professionals.

mmol/L
mmol/L
mmol/L

Total CO₂ on a chemistry panel

mmol/L

Only if your laboratory includes it

For the albumin-corrected gap

Used for the delta ratio; 12 is the usual figure

Anion gap
28 mmol/L
Delta ratio (ΔAG ÷ ΔHCO₃⁻)
1.33
  • Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) = 140 − (100 + 12) = 28 mmol/L.
  • Delta ratio = (28 − 12) ÷ (24 − 12) = 1.33. The ratio varies between patients with the same disorder, so read it with the whole picture (Kraut & Madias 2007).
  • Reference intervals differ between laboratories and analysers (often about 8–12 mmol/L without K⁺, lower on many modern analysers): compare with your own laboratory's range.

For healthcare professionals. A calculation, not an interpretation: verify against current guidelines, your laboratory's reference interval and the clinical picture.

How to use the anion gap calculator

  1. Enter sodium, chloride and bicarbonate (total CO₂) from the same sample, in mmol/L.
  2. Add potassium only if your laboratory's anion gap includes it, and albumin (g/dL or g/L) for the corrected gap.
  3. Read the gap, the albumin-corrected gap and, when the gap is raised, the delta ratio, then compare them with your laboratory's reference interval.

Worked example

Sodium 140, chloride 100 and bicarbonate 12 mmol/L give an anion gap of 140 − (100 + 12) = 28 mmol/L. With albumin 2.5 g/dL the corrected gap is 28 + 2.5 × (4.0 − 2.5) = 31.8 mmol/L: the low albumin was hiding about 4 mmol/L of the rise. The delta ratio is (31.75 − 12) ÷ (24 − 12) = 1.65.

What the anion gap is used for

Blood is electrically neutral, so the measured cations (sodium, sometimes potassium) and anions (chloride, bicarbonate) differ by the unmeasured anions: mostly albumin, plus phosphate, sulphate and organic acids. In a metabolic acidosis the gap helps separate acidosis from added acids, which raise the gap, from loss of bicarbonate or gain of chloride, which leave it unchanged. The result is one piece of information to read with the history, the blood gas and other tests.

Formulas

  • Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); with potassium, Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻)
  • Albumin-corrected gap = AG + 2.5 × (4.0 − albumin g/dL) (Figge J et al., Crit Care Med 1998;26:1807)
  • Delta ratio = (AG − 12) ÷ (24 − HCO₃⁻) (Kraut JA, Madias NE, Clin J Am Soc Nephrol 2007;2:162)

For healthcare professionals: a calculation, not a diagnosis. Reference intervals vary by laboratory. Related: the blood gas calculator shows Winters' formula and the expected compensation for each acid–base process.

Frequently asked questions

How is the anion gap calculated?

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), all in mmol/L (the same as mEq/L for these ions). With sodium 140, chloride 100 and bicarbonate 12 the gap is 140 − 112 = 28 mmol/L. Some laboratories add potassium: Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻), which makes the result about 4 higher, so always compare like with like.

Why correct the anion gap for albumin?

Albumin carries most of the unmeasured negative charge, so a low albumin lowers the gap and can hide a rise from acids such as lactate or ketones. Figge's correction adds 2.5 mmol/L for every 1 g/dL that albumin is below 4.0 g/dL (0.25 per g/L below 40 g/L). The slope is an approximation, and some authors question it.

What is a normal anion gap?

It depends on the analyser and the laboratory. Older references give about 8–12 mmol/L without potassium; many modern ion-selective electrode analysers report lower values. Use your own laboratory's reference interval rather than a number from a textbook.

What does the delta ratio show?

The delta ratio compares how far the gap has risen with how far bicarbonate has fallen: (AG − 12) ÷ (24 − HCO₃⁻). It is used when the gap is raised to look for a second acid–base process alongside a high-gap acidosis. Kraut and Madias showed the ratio varies widely between patients, so cut-offs quoted in different sources disagree and the ratio shouldn't be used on its own.

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