When acid is added to the body, the [H+] increases and the [HCO3-] decreases. In addition, the concentration of the anion, which is associated with the acid, increases. This change in the anion concentration provides a convenient way to analyze and help determine the cause of a metabolic acidosis by calculating what is termed the anion gap .
The anion gap is estimated by subtracting the sum of Cl- and HCO3- concentrations from the plasma Na concentration.
Na + Unmeasured cations = Cl- + HCO3- + Unmeasured anions
Anion gap = [Na] – ([Cl-] + [HCO3-])
The major unmeasured cations are calcium, magnesium, gamma globulins and potassium. The major unmeasured anions are negatively charged plasma proteins (albumin), sulphate, phosphates, lactate and other organic anions. The anion gap is defined as the quantity of anions not balanced by cations. This is usually equal to 12 ± 4 meq/L and is usually due to the negatively charged plasma proteins as the charges of the other unmeasured cations and anions tend to balance out.
If the anion of the acid added to plasma is Cl-. the anion gap will be normal (i.e. the
decrease in [HCO3-] is matched by an increase in [Cl-]). For example:
HCl + NaHCO3 → NaCl + H2CO3 → CO2 + H2O
In this setting, there is a meq. for meq. replacement of extracellular HCO3- by Cl- ; thus, there is no change in the anion gap, since the sum of Cl-] + [HCO3-] remains constant. This disorder is called a hyperchloremic acidosis, because of the associated increase in the Cl- concentration. GI or renal loss of HCO3- produces the same effect as adding HCl as the kidney in its effort to preserve the ECV will retain NaCl leading to a net exchange of lost HCO3- for Cl-.
In contrast, if the anion of the acid is not Cl- (e.g. lactate, β-hydroxybutyrate), the anion gap will increase (i.e. the decrease in [HCO3-] is not matched by an increase in the [Cl-] but rather by an increase in the [unmeasured anion]:
HA + NaHCO3 → NaA + H2CO3 → CO2 + H2O. where A- is the unmeasured anion.
Causes of elevated Anion gap acidosis is best remembered by the mnemonic KULT or the popular MUDPILES
M = Methanol