Calculate the serum anion gap from sodium, chloride and bicarbonate (or total CO₂) from the same testing time. If albumin is available, see an estimate adjusted for low albumin.
Anion Gap Calculator
Calculated anion gap
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), without potassium
Albumin-adjusted anion gap
When albumin is below 4.0 g/dL: adjusted gap = gap + 2.5 × (4.0 − albumin in g/dL)
Compared with your lab interval
The gap gives a clue about unmeasured ions. A higher value can occur with lactate, ketones, reduced kidney function or some toxic exposures; a low value can reflect low albumin or measurement error, among other causes. This number alone does not diagnose acidosis or its cause.
Limits
This calculation cannot establish blood pH or the full acid-base state. Interpret bicarbonate/total CO₂, blood gases, lactate, ketones, kidney function, medicines, exposures and laboratory methods together. Do not mix a potassium-inclusive formula with this formula’s reference interval.
What to review next
Check your laboratory interval and albumin, bicarbonate and creatinine from the same time. Discuss an unexpected value, including possible laboratory error, with a clinician. Severe symptoms or suspected toxic exposure require prompt care.
Questions for a visit
- How does this compare with this laboratory’s interval for the potassium-free formula?
- Should we review albumin, blood gas, lactate, ketones and kidney results together?
- Should any test be repeated or added?
Inputs are calculated in this browser and are not automatically saved or sent to the site. Printing or saving a PDF includes the values.
How to use it and example
For Na 140, Cl 104 and HCO₃ 20 mmol/L, the gap is 16. If albumin is 2.0 g/dL, the adjusted estimate is 16 + 2.5 × (4 − 2) = 21. Neither number is a diagnosis; interpret it with the laboratory interval and clinical context.
Compared with your lab interval
The gap gives a clue about unmeasured ions. A higher value can occur with lactate, ketones, reduced kidney function or some toxic exposures; a low value can reflect low albumin or measurement error, among other causes. This number alone does not diagnose acidosis or its cause.
Limits
This calculation cannot establish blood pH or the full acid-base state. Interpret bicarbonate/total CO₂, blood gases, lactate, ketones, kidney function, medicines, exposures and laboratory methods together. Do not mix a potassium-inclusive formula with this formula’s reference interval.
Frequently asked questions
Should I include potassium K⁺?
This tool uses the common potassium-free formula. Potassium-inclusive formulas produce different numbers and require different reference intervals.
Can I use total CO₂ instead of HCO₃⁻?
Total CO₂ in a routine serum chemistry panel largely reflects bicarbonate and is commonly used as a proxy. A blood-gas bicarbonate value may differ by method or timing; check the specimen context.
Related guides, logs and calculators
Kidney test guide · Lab result log · eGFR calculator · Health records
Sources
- Merck Manual Professional: Acid-Base Disorders
- Cleveland Clinic Journal of Medicine: Evaluating a low anion gap
- MedlinePlus: Anion Gap Blood Test
General health information, not a substitute for medical care. No independent clinician review is claimed. · Sources checked 2026-09-30