$\text{Anion}\text{Gap}\left(\text{AG}\right)=\left[{\text{Na}}^{+}\right]-\left[{\text{Cl}}^{-}+{\text{HCO}}_3^{-}\right]$

$\Delta \text{AG}=\Delta {\text{HCO}}_3^{-}$

Pure high anion gap metabolic acidosis

$\Delta \text{AG}<\Delta {\text{HCO}}_3^{-}$

High anion gap metabolic acidosis PLUS additional non-anion gap metabolic acidosis (i.e. RTA)

$\Delta \text{AG}>\Delta {\text{HCO}}_3^{-}$

High anion gap metabolic acidosis PLUS concurrent metabolic alkalosis (i.e. vomiting, hypercortisolism, hyperaldosteronism, contraction alkalosis)

$\Delta \text{AG}\gg \Delta {\text{HCO}}_3^{-}$

High anion gap metabolic acidosis PLUS primary respiratory alkalosis (i.e. severe lactic acidosis, septic shock, etc.)