Anion Gap Calculator
The anion gap helps sort out the cause of an acid-base problem. Enter sodium, chloride and bicarbonate (often reported as CO2 or total CO2) from the same blood test. Adding albumin gives a corrected gap, because low albumin hides a high gap.
| Result | Interpretation |
|---|---|
| Below 3 mEq/L | Low |
| 3 mEq/L to 12 mEq/L | Within the typical range |
| 12 mEq/L and above | High |
Source: 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); 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)
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Anion gap formula
Anion gap = sodium − (chloride + bicarbonate)
All values in mEq/L (the same as mmol/L). Bicarbonate is often listed as CO2 or total CO2 on a metabolic panel.
Worked example
Sodium 140, chloride 104, bicarbonate 24: 140 − (104 + 24) = 12 mEq/L.
What is a normal anion gap?
It depends on the lab’s equipment. Modern analyzers use ion-selective electrodes, which read chloride higher and give a lower gap than older machines did. Major textbooks quote different ranges:
| Textbook | Older methods | Ion-selective electrodes |
|---|---|---|
| Rose, Clinical Physiology of Acid-Base and Electrolyte Disorders | 7 to 13 | 3 to 11 |
| Emmett (UpToDate) | 7 to 13 | 3 to 9 |
| Salem and Batlle | 10 to 12 | 6 to 10 |
| Halperin | 8 to 16 | Not given |
This calculator uses about 3 to 11 mEq/L as the typical range. The normal range in one population can span 8 to 10 mEq/L, so it matters to compare with your own lab’s reference range.
Correcting for albumin
Low albumin makes the gap look smaller than it is: each 1 g/dL fall in albumin lowers the measured gap by about 2.5 mEq/L. The Figge correction adds that back:
Corrected anion gap = anion gap + 2.5 × (normal albumin − measured albumin)
This calculator uses 4.0 g/dL as normal albumin. If your lab’s normal albumin is higher, add another 2.5 mEq/L for each extra 1 g/dL.
For example, an anion gap of 10 with an albumin of 2.0 g/dL corrects to 10 + 2.5 × 2 = 15 mEq/L, which is high even though the measured gap looked normal.
Why potassium is left out
The original formula included potassium, but US textbooks now leave it out because it changes little in most people. When potassium is very high or very low, leaving it out can shift the gap by a few points.
FAQs
What does a high anion gap mean?
A gap above your lab’s normal range can point to a high anion gap metabolic acidosis. The cause needs to be worked out by your physician alongside your other results.
What does a low anion gap mean?
A low gap most often reflects a lab error or low albumin, so it is usually worth repeating before reading anything into it.
Should I use the corrected anion gap?
Yes, whenever albumin is low. Without the correction, a high gap can hide behind a normal-looking number.