anion gap is a calculated quantity used to assist in the Dx of metabolic acidosis
anion gap = ( [Na+]+[K+] ) − ( [Cl−]+[HCO3−] )
the average anion gap for healthy adults is 8-12 mEq/L
high anion gap
a high anion gap indicates that there is loss of HCO3− without a concurrent increase in Cl−.
electroneutrality is maintained by the elevated levels of anions like lactate, beta-hydroxybutyrate and acetoacetate, PO4−, and SO4−. These anions are not part of the anion-gap calculation and therefore a high anion gap results.
normal anion gap with metabolic acidosis
the drop in HCO3− is compensated for almost completely by an increase in Cl− and hence is also known as hyperchloremic acidosis.
low anion gap
most commonly due to hypoalbuminaemia as reduction in the negatively charged albumin results in an increase in bicarbonate and chloride ions to retain electroneutrality.
in hypoalbuminaemia the anion gap is reduced from between 1.5 and 2.5 mmol/L per g/dL decrease in serum albumin