Anion gap calculator (albumin-corrected)
Calculate the serum anion gap from sodium, chloride and bicarbonate, with optional potassium, and correct it for a low albumin with the Figge equation. Compare the result with your own lab's reference range.
How this anion gap calculator works
This anion gap calculator subtracts the measured anions, chloride and bicarbonate, from sodium, the main measured cation. The gap stands for anions the lab does not measure, mostly albumin and also phosphate, sulfate and organic acids. If you enter potassium, it is added to sodium. All electrolytes are in mmol/L, which equals mEq/L for these monovalent ions.
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), or (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻) with potassium
All values in mmol/L (mEq/L).
Albumin carries most of the unmeasured negative charge, so a low albumin lowers the anion gap and can hide a rise in other anions. Figge and colleagues measured this in 152 critically ill patients and found that the observed gap underestimates the unmeasured anions by about 0.25 mmol/L for each 1 g/L fall in albumin, which is 2.5 mmol/L per 1 g/dL. The calculator adds that back. Albumin in g/L is divided by 10 to give g/dL.
Albumin-corrected anion gap = anion gap + 2.5 × (4.0 − albumin)
Albumin in g/dL. The correction uses 4.0 g/dL as normal albumin; some references use 4.4 or 4.5 g/dL, which gives a corrected gap 1 to 1.25 mmol/L higher.
Anion gap results and what they suggest
The anion gap calculator compares the result with the lab range you enter, because there is no single normal anion gap. Kraut and Madias note that the normal value varies widely with the methods used to measure sodium, chloride and bicarbonate. Older continuous-flow analyzers gave mean gaps near 12 mmol/L, while ion-selective electrode analyzers give lower values, with a range often quoted as 3 to 11 mmol/L without potassium. That 3 to 11 range is the default here; replace it with your lab's own range.
| Result | Most common causes |
|---|---|
| High | Metabolic acidosis with unmeasured anions: lactate, ketoacids, toxic alcohols, kidney failure. Less often lab error, metabolic alkalosis, high phosphate or a paraprotein. |
| In range | No excess of unmeasured anions shown. A low bicarbonate with a normal gap fits a normal anion gap (hyperchloremic) metabolic acidosis, such as diarrhea or renal tubular acidosis. |
| Low | Most often lab error or a low albumin. Less often a paraprotein (such as IgG myeloma) or lithium, bromide or iodide intoxication. |
When the anion gap is less reliable
The anion gap is only as good as the three or four lab values behind it and the reference range used. The normal range in a population can span 8 to 10 mmol/L, so a real rise in unmeasured anions can still leave the gap inside the normal range, and a single reading is best compared with the patient's own earlier values. The albumin correction is an approximation derived in adult intensive care patients. Paraproteins, large changes in calcium or magnesium, and bromide or iodide (which some analyzers read as chloride) also shift the gap. The ratio of the rise in gap to the fall in bicarbonate (the delta ratio) varies between patients, so it should not be used alone to diagnose a mixed disorder.
Questions
What is a normal anion gap?
It depends on the lab's analyzer. Most labs now use ion-selective electrodes, with ranges often quoted as about 3 to 11 mmol/L without potassium, while older continuous-flow analyzers gave mean values near 12 mmol/L. Use the reference range printed on your lab's report.
Should I include potassium?
Most current references calculate the gap without potassium. If you include it, the gap rises by the potassium value, so compare it with a range for the gap with potassium. When you type a potassium, the calculator moves the default range from 3–11 to 7–15 mmol/L, adding a typical potassium of 4 mmol/L.
Why correct the anion gap for albumin?
Albumin is the main unmeasured anion. Each 1 g/dL fall in albumin lowers the gap by about 2.5 mmol/L, so in a patient with a low albumin a gap that looks normal can hide a high anion gap acidosis. The corrected gap estimates what the gap would be with a normal albumin.
Can I enter albumin in g/L?
Yes. Pick g/L under the albumin box. The calculator divides by 10 to get g/dL before it applies the correction. Leave albumin blank to see the uncorrected gap only.
Is my data sent anywhere?
No. The calculation runs in your browser, and the values you type are not sent or stored.
Sources
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807–1810. doi:10.1097/00003246-199811000-00019
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162–174. doi:10.2215/CJN.03020906
- 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. doi:10.2215/CJN.04040413