Anion gap calculator with the albumin-corrected gap
For healthcare professionals and students. Check results against your local protocols and clinical judgement; this tool does not give medical advice.
This anion gap calculator works out the serum anion gap from the electrolytes on a chemistry panel or blood gas — AG = Na⁺ − (Cl⁻ + HCO₃⁻) — with the option of adding potassium. It then applies the albumin correction of Figge and colleagues, compares the result with the reference range you enter, and calculates the delta ratio and delta gap used to look for a second acid–base disorder alongside a high anion gap acidosis.
All four ions are monovalent, so mmol/L and mEq/L are the same number and no unit conversion is needed; albumin can be entered in g/dL or g/L. The working is shown line by line. Nothing leaves your browser.
How to use the Anion Gap Calculator
- Enter sodium, chloride and bicarbonate or total CO₂ in mmol/L.
- Tick Include potassium in the gap if your laboratory reports the gap that way.
- Enter albumin to get the corrected gap, or leave it blank; adjust the normal albumin if your protocol uses another value.
- Set your laboratory’s anion gap range and normal bicarbonate.
- Read the corrected gap, the delta ratio and delta gap, and the interpretation notes with their sources.
Anion Gap Calculator features
ABG or chemistry panel
Uses calculated HCO₃⁻ from a blood gas or total CO₂ from a chemistry panel; the tool says which is which.
Potassium option
Na + K − (Cl + HCO₃), with a reminder that the reference range must include potassium too.
Albumin correction
Figge’s 0.25 mmol/L per g/L (2.5 per g/dL) below a normal albumin you can change.
Your laboratory’s range
Classifies the gap against the range you enter rather than a fixed textbook figure.
Delta ratio and delta gap
With Wrenn’s ±6 thresholds and Rastegar’s caution quoted beside the number.
Formulas and worked example
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); with potassium, Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻). The gap is not a real deficit of anions: it reflects unmeasured anions — mainly albumin and phosphate — minus unmeasured cations, as reviewed by Kraut JA and Madias NE, Clin J Am Soc Nephrol 2007;2(1):162–174.
Albumin-corrected anion gap = AG + 0.25 × (normal albumin − measured albumin), albumin in g/L, or 2.5 per g/dL. Figge J, Jabor A, Kazda A and Fencl V, Crit Care Med 1998;26(11):1807–1810, found that each 1 g/L fall in albumin made the observed gap underestimate the unmeasured anions by 0.25 mEq/L. The paper writes “[normal albumin]” rather than a fixed number; 4.0 g/dL (40 g/L) is used here by default and 4.4 g/dL is also seen.
Delta ratio = (AG − normal AG) ÷ (normal HCO₃⁻ − HCO₃⁻) and delta gap = ΔAG − ΔHCO₃⁻. Worked example (used in the self-test): Na 138, Cl 100, HCO₃ 12 gives a gap of 26; albumin 3.2 g/dL adds 0.25 × 8 = 2, so the corrected gap is 28. With a normal gap of 12 and normal bicarbonate 24, the delta ratio is 16 ÷ 12 = 1.33 and the delta gap +4.
Which normal range?
The traditional 8–16 mmol/L range dates from older analysers. Winter SD et al., Arch Intern Med 1990;150(2):311–313, found that ion-selective electrodes shifted it down — to 3–11 mmol/L in one of their laboratories — mainly because measured chloride rose. Kraut and Madias note that the normal value “can vary widely”. Use the range your own laboratory publishes; the calculator takes its mid-point as the normal gap for the delta calculations.
| Delta gap | Interpretation (Wrenn 1990) |
|---|---|
| Greater than +6 | A metabolic alkalosis is usually present: the rise in AG is more than the fall in HCO₃ |
| Less than −6 | A hyperchloraemic acidosis is usually present: the rise in AG is less than the fall in HCO₃ |
| −6 to +6 | Neither threshold is reached |
Limits of the delta ratio
Wrenn K, “The delta (Δ) gap”, Ann Emerg Med 1990;19(11):1310–1313, introduced the delta gap. Rastegar A, J Am Soc Nephrol 2007;18(9):2429–2431, showed that although the mean ΔAG/ΔHCO₃ ratio in selected patients is close to 1, its range is wide, so it “should be used cautiously” to diagnose mixed disorders. Read it together with the history, pH, PCO₂ and, where relevant, the osmolal gap.
Anion Gap Calculator FAQ
How do you calculate the anion gap?
Subtract the sum of chloride and bicarbonate from sodium: AG = Na − (Cl + HCO₃). For Na 140, Cl 104 and HCO₃ 24 the gap is 12 mmol/L. The anion gap calculator above also shows the version with potassium.
Can I use the CO₂ on a metabolic panel instead of bicarbonate?
Yes. The “CO₂” on a chemistry panel is total CO₂, which is almost all bicarbonate and is what most laboratories use for the gap. A blood gas reports a calculated bicarbonate; the two usually differ by a small amount.
Is the anion gap in mmol/L or mEq/L?
Either — for sodium, potassium, chloride and bicarbonate one mmol/L equals one mEq/L, so the number is the same.
Why correct the anion gap for albumin?
Albumin is the main unmeasured anion. When it is low, the gap falls and can look normal despite a high anion gap acidosis. Figge’s correction adds 2.5 mmol/L for every 1 g/dL that albumin is below normal.
What does a delta ratio of 1 to 2 mean?
Values around 1 are expected in a pure high anion gap acidosis. Wrenn’s delta gap thresholds of ±6 are given beside the ratio, and Rastegar warns that the ratio varies widely between patients.
What causes a low anion gap?
Kraut and Madias list laboratory error and low albumin as the most common causes, then paraproteins and lithium, bromide or iodide intoxication. A related tool is the corrected calcium calculator.