Anion Gap Calculator
Anion gap from sodium, chloride and bicarbonate, with the albumin correction and species ranges.
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). Veterinary labs usually include potassium: Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻). Low albumin lowers the gap, so add 2.5 mEq/L for every 1 g/dL of albumin below normal. A high anion gap points to unmeasured acids such as lactate, ketones, uremic acids or ethylene glycol.
Free, runs in your browser. Nothing you enter is uploaded.
How to use it
- Choose human, dog or cat.
- Enter sodium, chloride and bicarbonate (or total CO₂); potassium is used for dogs and cats.
- Add albumin to get the albumin-corrected gap.
What you enter
- Sodium, chloride and bicarbonate (mEq/L)
- Potassium for dogs and cats
- Albumin (optional)
What you get
- Anion gap
- Albumin-corrected anion gap
- High, low or within the species range
Formula
AG = Na⁺ − (Cl⁻ + HCO₃⁻) | with K⁺: AG = Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻) | corrected AG = AG + 2.5 × (normal albumin − albumin, g/dL)
Electrolytes in mEq/L, albumin in g/dL. The calculator uses 4.0 g/dL as normal albumin.
Worked example
Input: Na 140, Cl 104, HCO₃ 24 mEq/L, albumin 2.5 g/dL (human)
Result: AG 12 mEq/L; corrected AG 15.8 mEq/L
The raw gap looks normal, but with low albumin the corrected gap is near the top of the range, so a hidden high-gap acidosis is worth checking.
Questions
What is the formula for anion gap?
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), in mEq/L. Some labs, and most veterinary labs, add potassium: Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻), which makes the normal range about 4 mEq/L higher.
Why correct the anion gap for albumin?
Albumin is the main unmeasured anion. When albumin is low the gap falls, and a real high-gap acidosis can look normal. The corrected gap adds 2.5 mEq/L for each 1 g/dL of albumin below normal.
What causes a high anion gap?
The common causes are lactic acidosis, ketoacidosis (diabetic, alcoholic or starvation), kidney failure and toxins such as ethylene glycol (antifreeze), methanol and salicylates.
What is a normal anion gap in dogs and cats?
With potassium included, typical ranges are about 12–24 mEq/L in dogs and 13–27 mEq/L in cats. Ranges vary between analyzers, so use your lab's range when it has one.
Limitations
- Reference ranges differ between analyzers.
- The albumin correction is an approximation and is less reliable in critically ill patients.
References
- Figge J, Jabor A, Kazda A, Fencl V (1998) Critical Care Medicine 26(11):1807-1810.
- Kraut JA, Madias NE (2007) Clinical Journal of the American Society of Nephrology 2(1):162-174.
More Veterinary
- Chocolate Toxicity Calculator
- Dog Weight Loss Calculator
- Dog IV Fluid Rate Calculator
- Dog Pregnancy / Whelping Calculator
- Feline Grimace Scale Calculator
- Corrected Calcium Calculator
- IRIS CKD Staging Calculator (Cats & Dogs)
All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.