Anion gap
In this case, the values in mEq/L are equal to those in mmol/L. If your results are in mmol/L, type in these values.

This calculator helps you estimate the anion gap based on sodium (Na⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻) levels measured in blood serum. In advanced mode, there is an option to input potassium (K⁺) and albumin levels to get a more accurate result or to see the anion gap corrected for albumin. Read on to understand the anion gap calculation, to learn what is anion gap and what is a normal anion gap.

If this is helpful to you, you might like our GFR calculator or corrected calcium calculator.

Please note that this calculator estimates the anion gap for serum anions. For urine anion gap, check out our urine anion gap calculator.

What is anion gap?

In blood serum, the total number of cations (positive ions) should be equal to the total number of anions (negative ions), so that the overall electrical charge is neutral. However, routine tests do not measure all types of ions. The anion gap is the difference between the measured cations and anions. It is representative of how many ions are not accounted for by the lab measurements.

In medicine, anion gap calculation is required when attempting to identify the cause of acidosis, which is a lower than normal pH in the blood. It is usually calculated from sodium, chloride and bicarbonate concentrations in the blood serum.

How is the anion gap calculated?

To estimate the anion gap, it is necessary to measure electrolyte levels in the blood serum.

  • Daily practice:
    It is the difference between the sodium (Na⁺) cations and chloride (Cl⁻) and bicarbonate (HCO₃⁻) anions:
    AG = [Na⁺] − ([Cl⁻] + [HCO₃⁻])

  • With potassium:
    Potassium concentrations are very low in comparison to other ions, so they're omitted from the regular calculation. You can find the result more accurate when including potassium, though. Then, the equation looks like this:
    AG = [Na⁺] + [K⁺] − ([Cl⁻] + [HCO₃⁻])

What is a normal anion gap?

The reference range when potassium is omitted is 3-11 mEq/L. When including potassium in the equation, it's 5-16 mEq/L.

Interpretation of the outcome:

  • High anion gap >11 mEq/L (or >16 mEq/L with potassium)
    A high anion gap indicates high anion gap acidosis, for example, lactic acidosis or ketoacidosis.
  • Normal anion gap 3-11 mEq/L (or 5-16 mEq/L with potassium)
    In patients with normal anion gap acidosis, the primary pathology is a drop in HCO₃⁻ and an increase of Cl⁻. It is called hyperchloremic acidosis.
  • Low anion gap - <3 mEq/L (or <5 mEq/L with potassium)
    A low anion gap is frequently caused by hypoalbuminemia.

Why is anion gap corrected for albumin?

Low albumin levels (hypoalbuminemia), common in critically ill patients, can mask a mild elevation of the anion gap. For the correct interpretation of the anion gap it is crucial to correct it for serum albumin with Figge-Jabor-Kazda-Fencl equation:

Corrected AG = AG + 2.5 x ([Normal Albumin] - [Measured Albumin])

If you're interested to know more about the correction for albumin, read this article on NCBI.

Małgorzata Koperska, MD