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. Calculation 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). With potassium The anion gap is calculated by subtracting the serum concentrations of chloride and bicarbonate (anions) from the concentrations of sodium and potassium (cations): = ([Na+] + [K+]) − ([Cl−] + [HCO]) = 20 mEq/L Without potassium 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: = [Na+] - ([Cl−] + [HCO]) Normal AG = 8-16 mEq/L Expressed in words, the equation is: Anion Gap = Sodium - (Chloride + Bicarbonate) which is logically equivalent to: Anion Gap = (the most prevalent cation) minus (the sum of the most prevalent anions) (Bicarbonate may also be referred to as "total CO2" or "carbon dioxide".) Uses Calculating the anion gap is clinically useful because it helps in the differential diagnosis of a number of disease states. 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 representative of how many ions are not accounted for by the lab measurements used in the calculation. These "unmeasured" ions are mostly anions, which is why the value is called the "anion gap." By definition, only the cations sodium (Na+) and potassium (K+) and the anions chloride (Cl−) and bicarbonate (HCO) are used to