Anion Gap

What is Anion Gap?
The term anion gap (AG) represents the concentration of all the unmeasured anions in the plasma. The negatively charged proteins account for about 10% of plasma anions and make up the majority of the unmeasured anion represented by the anion gap under normal circumstances. The acid anions (i.e. lactate, acetoacetate, sulphate) produced during a metabolic acidosis are not measured as part of the usual laboratory biochemical profile. The H+ produced reacts with bicarbonate anions (buffering) and the CO2 produced is excreted via the lungs (respiratory compensation). The net effect is a decrease in the concentration of measured anions (i.e. HCO3-) and an increase in the concentration of unmeasured anions (the acid anions) so the anion gap increases.
AG is calculated from the following formula:
Anion gap = [Na+] – [Cl-] – [HCO3-] Reference range is 8 to 16 mEq/l. An alternative formula which includes K+ is sometimes used particularly by Nephrologists. In Renal Units, K+ can vary over a wider range and have more effect on the Anion Gap.
The alternative formula is:
Anion Gap = [Na+] + [K+] – [Cl-] – [HCO3-] The reference range is slightly higher with this alternative formula. The [K+] is low relative to the other three ions and it typically does not change much so omitting it from the equation doesn’t have much clinical significance.