The anion gap (AG or AGAP) is a value calculated from the results of multiple individual medical lab tests. It may be reported with the results of an electrolyte panel, which is often performed as part of a comprehensive metabolic panel.

The anion gap is the quantity difference between cations (positively charged ions) and anions (negatively charged ions) in serum, plasma, or urine. The magnitude of this difference (i.e., "gap") in the serum is calculated to identify metabolic acidosis. If the gap is greater than normal, then high anion gap metabolic acidosis is diagnosed.

The term "anion gap" usually implies "serum anion gap", but the urine anion gap is also a clinically useful measure.

The anion gap is a calculated measure. It is computed with a formula that uses the results of several individual lab tests, each of which measures the concentration of a specific anion or cation.

The concentrations are expressed in units of milliequivalents/liter (mEq/L) or in millimoles/litre (mmol/L).

The anion gap is calculated by subtracting the serum concentrations of chloride and bicarbonate (anions) from the concentrations of sodium and potassium (cations):

Because potassium concentrations are very low, they usually have little effect on the calculated gap. Therefore, omission of potassium has become widely accepted. This leaves the following equation:

Normal AG = 8-16 mEq/L