UAG = Urine Na + Urine K − Urine Cl. This is an indirect estimate of urine ammonium (NH4+). NH4+ is excreted with Cl−, so high NH4+ excretion increases urine Cl relative to Na+K, making the UAG negative.

A negative UAG (typically −20 to −50 mEq/L) indicates that the kidneys are appropriately excreting ammonium in response to acidosis. This suggests an extrarenal cause of NAGMA, most commonly diarrhea or other GI bicarbonate losses.

A positive UAG (>0 mEq/L) in the setting of NAGMA suggests impaired renal ammonium excretion, pointing to renal tubular acidosis (RTA). Type 1 (distal) RTA has impaired H+ secretion, Type 4 (hyperkalemic) RTA has aldosterone deficiency or resistance. Further workup with urine pH and serum potassium helps differentiate.