Acid-base · Lab interpretation
Anion gap calculator.
Enter sodium, chloride and bicarbonate to get the serum anion gap. Add albumin to see the albumin-corrected anion gap, which matters when albumin is low.
Sodium 140, chloride 104 and bicarbonate 24 mmol/L give an anion gap of 12. With an albumin of 2.5 g/dL the albumin-corrected gap is 16.75. Enter your own values below.
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How the anion gap and its albumin correction work
Anion gap = sodium - (chloride + bicarbonate), in mmol/L. Some laboratories and papers add potassium to the sodium side; the optional box does that, and the label in the result tells you which version you got. The gap is the difference between the measured cations and the measured anions, so it stands in for the unmeasured anions in blood. A bicarbonate that drops without an equal rise in chloride makes the gap rise, which is why it is used when working out the cause of a low bicarbonate.
The albumin correction exists because albumin is itself the main unmeasured anion. When albumin is low the gap shrinks even if other unmeasured anions are present, which can hide a real problem. The Hatherill 2002 paper quotes the Figge equation as corrected AG = AG + 0.25 × (44 - albumin), with albumin in g/L. Because 1 g/dL is 10 g/L, that is the same as adding 2.5 for each 1 g/dL that albumin is below 4.4. The calculator converts g/dL to g/L for you and shows how much the correction added.
Worked example
Sodium 140, chloride 104, bicarbonate 24: 140 - (104 + 24) = 12. With albumin 2.5 g/dL (25 g/L) the correction is 0.25 × (44 - 25) = 4.75, so the corrected gap is 16.75. A second case: sodium 140, chloride 100, bicarbonate 12 gives a gap of 28 and, at a normal albumin of 4 g/dL (40 g/L), a corrected gap of 29. The size of the correction depends only on albumin, not on the electrolytes.
The evidence on correction comes from critically ill children. In the Hatherill study of children in shock, the mean uncorrected gap was 15.0 mEq/L against a corrected 19.9 mEq/L, and in the Durward study of critically ill children correction raised the share of samples with a gap above 18 mEq/L from 28% to 44%. That is a specific population, and both papers describe the equation as Figge's, not their own. This page gives no normal range, because the papers opened for it use different cut-offs and laboratories differ; read the result against the interval printed on your own report.
Common mix-ups
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Often assumedLow albumin does not matter when reading an anion gap.
What the source saysIn a study of children in shock the mean anion gap was 15.0 mEq/L before correction and 19.9 mEq/L after correcting for albumin with the Figge equation, which the authors used to look for occult tissue anions. [1]
Checked against these sources on .
Frequently asked questions
What is the anion gap formula?
Anion gap = sodium - (chloride + bicarbonate), in mmol/L. Some labs add potassium to the sodium side, and this calculator lets you do that with the optional potassium box.
Why correct the anion gap for albumin?
Albumin is the main unmeasured anion, so a low albumin lowers the gap and can mask a raised gap from other unmeasured anions. The Figge correction adds back 0.25 x (44 - albumin in g/L).
How do I convert albumin from g/dL to g/L?
Multiply by 10. An albumin of 2.5 g/dL is 25 g/L. The calculator does this when you choose g/dL.
What is a normal anion gap?
It depends on the laboratory, the analyser and whether potassium is included, so there is no single number. Use the reference interval printed on your lab report. This calculator does not label results as normal or abnormal.
Does the albumin correction apply to everyone?
The studies behind it were in critically ill children. How well it generalises to other patients is a question for your clinician, and the corrected value is an aid to interpretation, not a diagnosis.
Sources
CheckedSources retrieved
- Hatherill M et al: Correction of the anion gap for albumin in order to detect occult tissue anions in shock (Arch Dis Child, 2002)
- Durward A et al: Hypoalbuminaemia in critically ill children: incidence, prognosis, and influence on the anion gap (Arch Dis Child, 2003)
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Not medical advice
This calculator does arithmetic on the numbers you type and does not diagnose or treat anything. Results depend on the quality of the inputs and the equation used. Lab values, drug doses and ECG readings must be interpreted by a clinician who knows the patient; contact your care team or emergency services for anything urgent.