What Is the Anion Gap?
The anion gap is a calculated value used in electrolyte and acid-base review. It compares the major measured positive ion, sodium, with the major measured negative ions, chloride and bicarbonate. The difference helps clinicians look for unmeasured acids or other shifts in blood chemistry.
Anion gap is most often discussed when evaluating metabolic acidosis, but it should not be interpreted alone. A complete review usually includes pH, bicarbonate, carbon dioxide, kidney function, lactate, ketones, and clinical context. For related electrolyte unit work, use the mEq/L to mmol/L Conversion Calculator or the mmol/L to mEq/L Conversion Calculator.
Anion Gap Formula
The most common serum anion gap formula excludes potassium because potassium contributes only a small amount to the extracellular cation pool.
Anion Gap = Sodium - (Chloride + Bicarbonate) Some clinicians include potassium, especially in older references or specific institutional protocols.
Anion Gap = Sodium + Potassium - (Chloride + Bicarbonate) Albumin-Corrected Anion Gap
Albumin is an important unmeasured anion. When albumin is low, the anion gap may look falsely normal even when clinically important unmeasured acids are present. This is why many clinicians correct the anion gap for albumin in sick or hospitalized patients.
Corrected AG = AG + 2.5 x (4.0 - albumin in g/dL) Example: if the anion gap is 12 and albumin is 2.4 g/dL, the corrected anion gap is 12 + 2.5 x (4.0 - 2.4) = 16 mEq/L. That correction can change how the result is interpreted.
Step-by-Step Example
Suppose a chemistry panel shows sodium 138 mEq/L, chloride 100 mEq/L, and bicarbonate 14 mEq/L.
AG = 138 - (100 + 14) = 24 mEq/L An anion gap of 24 is elevated for most modern reference ranges. In a patient with low bicarbonate, that pattern may suggest high anion gap metabolic acidosis, but the cause must be evaluated clinically.
Anion Gap Reference Ranges
Normal ranges vary by laboratory and analyzer method. Many modern labs report a standard anion gap reference range around 3-11 or 4-12 mEq/L when potassium is excluded. Older references and potassium-included formulas often use higher ranges, such as about 12-16 mEq/L.
| Result pattern | Common meaning |
|---|---|
| Low anion gap | May occur with low albumin, lab artifact, high unmeasured cations, or paraproteins. |
| Typical range | Often around 3-11 or 4-12 mEq/L without potassium, depending on the lab. |
| High anion gap | May occur with lactic acidosis, ketoacidosis, kidney failure, toxins, or other unmeasured acids. |
Anion Gap Quick Reference Chart
These examples use the standard formula without potassium.
| Na | Cl | HCO3 | Anion Gap | Context |
|---|---|---|---|---|
| 140 | 104 | 24 | 12 | Typical example using Na - (Cl + HCO3) |
| 138 | 100 | 14 | 24 | High anion gap pattern |
| 140 | 110 | 18 | 12 | Normal gap with lower bicarbonate |
| 136 | 108 | 25 | 3 | Low anion gap example |
| 142 | 102 | 12 | 28 | Marked high anion gap example |
When This Calculator Is Useful
This calculator is useful when reviewing a basic metabolic panel, comprehensive metabolic panel, emergency department labs, ICU labs, or blood gas chemistry. It can help standardize the math while you compare sodium, chloride, bicarbonate, and albumin from the same clinical moment.
If the case also involves kidney markers, the Creatinine mg/dL to µmol/L Calculator can help compare renal lab units. For calcium and other electrolyte conversions, see the Calcium Conversion Calculator.
Frequently Asked Questions
What is the standard anion gap formula?
The standard formula is sodium minus chloride plus bicarbonate: AG = Na - (Cl + HCO3). Some clinicians include potassium, but the potassium-free formula is most common.
What is a normal anion gap?
A typical modern reference range is often around 3-11 or 4-12 mEq/L when potassium is excluded. Always use the range printed on your lab report because methods differ between laboratories.
Why correct anion gap for albumin?
Low albumin lowers the measured anion gap. Albumin correction helps avoid missing an elevated gap in patients with hypoalbuminemia.
Is mEq/L the same as mmol/L for anion gap electrolytes?
For common monovalent electrolytes such as sodium, chloride, and bicarbonate, the numeric values in mEq/L and mmol/L are the same. For charged divalent ions such as calcium and magnesium, valence matters.
Can this calculator diagnose metabolic acidosis?
No. It calculates a value that can support acid-base review, but diagnosis requires clinical evaluation, pH, blood gas interpretation, kidney function, lactate, ketones, and other testing as appropriate.