Anion Gap Calculator

In blood serum, the total number of cations (positive ions) should equal the total number of anions (negative ions), so 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 represents 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. Are you interested in learning the chemical names of chemical compounds or how to calculate them? Then visit our chemical name calculator to learn more.

Measuring electrolyte levels in the blood serum is necessary to estimate the anion gap.

• 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 compared to other ions, so they're omitted from the standard calculation. You can find the result is more accurate when including potassium, though. Then, the equation looks like this:

  AG = [Na⁺] + [K⁺] − ([Cl⁻] + [HCO₃⁻])

When potassium is omitted, the reference range 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 regular 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):

  Hypoalbuminemia frequently causes a low anion gap.

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 the Figge-Jabor-Kazda-Fencl equation:

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

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