How do you calculate the anion gap?
Subtract chloride and bicarbonate from sodium. Blood is electrically neutral, but routine tests measure only some of the charged particles. The difference stands for anions that are present but not measured: mostly albumin, phosphate, sulphate and organic acids.
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
Albumin-adjusted gap = anion gap + 0.25 × (40 − albumin g/L)
All electrolytes are in mmol/L, which for these ions is the same as mEq/L. Both results are shown to up to one decimal place, without trailing zeros.
How to use the calculator
- Enter Sodium (Na⁺), Chloride (Cl⁻) and Bicarbonate (HCO₃⁻) from the same sample, in mmol/L. A blood gas often gives you all three.
- For the adjusted value, choose the Albumin unit (g/L or g/dL) and fill in Albumin (optional). Leave it empty to skip the adjustment.
- Read the Anion gap and, if you entered albumin, the Albumin-adjusted anion gap.
Worked examples
Example 1 – unremarkable values. Na⁺ 140, Cl⁻ 104, HCO₃⁻ 24 mmol/L. 140 − (104 + 24) = 140 − 128 = 12 mmol/L.
Example 2 – low bicarbonate. Na⁺ 134, Cl⁻ 98, HCO₃⁻ 10 mmol/L, as you might see in diabetic ketoacidosis. 134 − (98 + 10) = 134 − 108 = 26 mmol/L. Bicarbonate has fallen without a matching rise in chloride, so the gap is wide.
Example 3 – low albumin. Na⁺ 138, Cl⁻ 105, HCO₃⁻ 22 mmol/L, albumin 20 g/L. Anion gap: 138 − (105 + 22) = 11 mmol/L. Adjusted: 11 + 0.25 × (40 − 20) = 11 + 5 = 16 mmol/L. A gap that looked normal is raised once you allow for albumin. If your report gives albumin as 2.0 g/dL, pick g/dL: the calculator converts it to 20 g/L and the result is the same.
Albumin adjustment at a glance
Amount added to the anion gap at different albumin levels.
| Albumin | Added to the gap |
|---|---|
| 40 g/L (4.0 g/dL) | 0 mmol/L |
| 35 g/L (3.5 g/dL) | +1.25 mmol/L |
| 30 g/L (3.0 g/dL) | +2.5 mmol/L |
| 25 g/L (2.5 g/dL) | +3.75 mmol/L |
| 20 g/L (2.0 g/dL) | +5 mmol/L |
| 15 g/L (1.5 g/dL) | +6.25 mmol/L |
Common mistakes
- Comparing with the wrong reference range. A formula with potassium gives a gap about 4 mmol/L higher, and ranges differ between analysers.
- Ignoring albumin in unwell patients, in whom low albumin is common.
- Wrong albumin unit. 2.5 g/dL entered as g/L adds almost 9.4 mmol/L instead of 3.75.
- Mixing sources. Bicarbonate from a blood gas and sodium and chloride from the main laboratory are measured differently. When possible, it’s best to use values from one sample and one analyser.
Limits of the anion gap
- A normal gap does not rule out an acid such as lactate, especially when albumin is low or the rise is small.
- Unmeasured cations (calcium, magnesium, lithium, some paraproteins) can lower the gap.
- Laboratory interference, for example from bromide or very high lipids, can distort chloride or sodium.
- The anion gap is a pointer, not a diagnosis. It is best read with the pH, the history and other tests.
The arterial blood gas guide shows where the anion gap fits in the full reading. If you suspect a toxic alcohol, pair it with the osmolal gap. Albumin also changes total calcium: see the corrected calcium calculator.
Frequently asked questions
What is the anion gap formula?
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), with all values in mmol/L. This calculator leaves potassium out, which is the most common convention. If your laboratory includes potassium, its gap and its reference range run about 4 mmol/L higher, so it’s best to compare like with like.
What is a normal anion gap?
Roughly 8–12 mmol/L without potassium is the figure most often quoted, but it depends on the analyser. Modern laboratory analysers and blood gas machines often have lower reference ranges. It’s advisable to use the range of the instrument that measured the sample, not a textbook number.
Why adjust the anion gap for albumin?
Because albumin is the main unmeasured anion in plasma. When albumin falls, the gap falls by roughly 2.5 mmol/L for every 10 g/L (1 g/dL). A gap raised by lactate or ketones can then hide inside a “normal” number, which the adjustment brings back into view.
What is the albumin adjustment used here?
Adjusted anion gap = anion gap + 0.25 × (40 − albumin in g/L), following Figge and colleagues. If you choose g/dL as the albumin unit, the calculator multiplies your value by 10 first. Leave the albumin field empty and you get the plain anion gap only.
What does a raised anion gap suggest?
In a metabolic acidosis, a raised gap points to extra unmeasured acids: lactate, ketones, acids retained in kidney failure, or some poisons. A normal-gap acidosis usually comes from losing bicarbonate or gaining chloride. Either way, the gap is read together with the full clinical picture.
Can the anion gap be negative or very low?
Rarely, and it usually means something is off. A very low or negative gap can come from a laboratory error, severe hypoalbuminaemia, abnormal proteins such as in myeloma, or substances like bromide that interfere with the chloride measurement. Repeating the test before drawing conclusions is usually advisable.