Anion gap calculator and normal range
Short answer: The anion gap is sodium minus chloride and bicarbonate: Na − (Cl + HCO₃). With modern analysers it is usually about 3 to 11 mmol/L, but labs print ranges from 3–11 to 8–16, so your report’s range comes first. A low albumin lowers the gap, so the albumin-corrected gap adds 2.5 for every 1 g/dL that albumin is below 4.
Free calculator
Anion gap calculator
Anion gap = sodium − (chloride + bicarbonate)
Sources: Kraut JA, Nagami GT. The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved? Clin J Am Soc Nephrol 2013;8(11):2018-2024.
mmol/L is the same number as mEq/L
Sometimes printed as "CO2" or "bicarb"
Optional: corrects the gap for a low albumin
Your result
Enter the numbers from your lab report to see your result.
The anion gap is read by doctors alongside the rest of the blood gas or chemistry panel, usually when someone is unwell. A number outside the range on its own, with no symptoms, is often a lab variation.
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A pointer, not a diagnosis. Talk to a doctor about your result, and sooner if you feel unwell.
How to read the result
- Low (below 3)< 3mmol/LLower than usual. The most common cause is a low albumin; others are a lab error and, rarely, high calcium, magnesium or lithium, or some blood proteins (paraproteins). Ask a doctor about it if it stays low.
- Typical (3 to 11)3 – 12mmol/LIn the range modern lab analysers usually give.
- 12 to 1612 – 17mmol/LAbove the 3–11 range of modern analysers, but inside the 8–16 range many labs still print. Your lab’s own range decides. On its own, with no symptoms, it rarely means anything.
- Raised (17 or above)≥ 17mmol/LRaised on any lab’s range. A high anion gap points to an acid building up in the blood, for example from uncontrolled diabetes (ketones), kidney failure, lactic acid in a severe illness or some poisonings. If you feel unwell, get medical care now.
Your lab’s range
Lab ranges for the anion gap vary more than for most tests, because they depend on the analyser: about 3–11 mmol/L with today’s ion-selective electrodes, 8–16 or 7–13 on older methods, and about 4 higher when the lab adds potassium. Read your result against the range on your own report.
How it is worked out
Anion gap = sodium − (chloride + bicarbonate)
The anion gap is read by doctors alongside the rest of the blood gas or chemistry panel, usually when someone is unwell. A number outside the range on its own, with no symptoms, is often a lab variation.
Sources
- Kraut JA, Nagami GT. The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved? Clin J Am Soc Nephrol 2013;8(11):2018-2024.
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol 2007;2(1):162-174.
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med 1998;26(11):1807-1810.
The markers it uses
Questions
- What is a normal anion gap?
- About 3 to 11 mmol/L on modern analysers, and 8 to 16 on many older ones; some labs add potassium, which makes it about 4 higher. That is why the range printed on your own report is the one to use.
- How do you calculate the anion gap?
- Sodium − (chloride + bicarbonate), all in mmol/L (the same number as mEq/L). Sodium 140, chloride 104 and bicarbonate 24 give 140 − 128 = 12. Bicarbonate is often printed as total CO₂.
- What does a high anion gap mean?
- An unmeasured acid in the blood: ketones in uncontrolled diabetes or starvation, lactic acid in a severe illness, kidney failure, or poisons such as methanol or ethylene glycol. A doctor reads it with the blood gas and the symptoms. If you feel unwell, seek care now.
- Why correct the anion gap for albumin?
- Albumin is the biggest unmeasured negative charge, so a low albumin lowers the gap and can hide a raised one. The correction adds 2.5 mmol/L for every 1 g/dL that albumin is below 4.0 (Figge, 1998).
More calculators for the same system
A pointer, not a diagnosis. Talk to a doctor about your result, and sooner if you feel unwell. Reference ranges are general-population guidance compiled from common clinical laboratories, not a diagnosis. Lab-to-lab ranges differ. Always read your results with your doctor.