Anion Gap Calculator with Albumin Correction
Calculate the serum anion gap and, if you enter albumin, the albumin-corrected gap.
✓ Formulas cited from the published literature
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About this calculator
The anion gap is the difference between measured positive ions (sodium) and measured negative ions (chloride and bicarbonate). It helps narrow the cause of metabolic acidosis: a high gap points to added acids, a normal gap to bicarbonate loss or chloride gain.
How it works
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
Albumin-corrected AG = AG + 2.5 × (4.0 − albumin in g/dL)
Potassium is excluded here, which is the most common convention. Low albumin lowers the gap and can hide a true elevation, hence the correction.
How to read your result
Reference intervals depend on your lab's analyser. Older flame-photometry ranges (8–16) differ from modern ion-selective electrodes (about 3–11). Always compare with your lab's range.
Worked example
Sodium 140, chloride 100, bicarbonate 15 mmol/L, albumin 3.0 g/dL.
| Anion gap | 25 mmol/L Elevated (typical 8–12; many labs now report 3–11) |
| Albumin-corrected anion gap | 27.5 mmol/L AG + 2.5 × (4.0 − albumin g/dL) |
Frequently asked questions
What is a normal anion gap?
About 8–12 mmol/L in most textbooks, but many modern labs report 3–11. Check your lab range.
What causes a high anion gap?
Lactic acidosis, ketoacidosis, kidney failure, and toxic ingestions such as methanol, ethylene glycol and salicylates.
Why correct for albumin?
Albumin is an unmeasured anion. A low albumin shrinks the gap by about 2.5 mmol/L for each 1 g/dL fall and can mask real acidosis.
Limitations and when not to use
Adult formula. Interpretation depends on arterial blood gas and clinical context.
Sources and references
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med 1998;26:1807–1810.
Last updated: 2026-10-09