Urine anion gap is calculated by subtracting the urine concentration of chloride (anions) from the concentrations of sodium plus potassium (cations):[3]
In contrast to the serum anion gap equation, the bicarbonate is excluded.
[4] Determining the cause of a metabolic acidosis that lacks a serum anion gap often depends on determining whether the kidney is appropriately excreting acid.
[5] Urine NH4+ is difficult to measure directly, but its excretion is usually accompanied by the anion chloride.
A negative urine anion gap can be used as evidence of increased NH4+ excretion.