UAG — Distal RTA Workup
The Urine Anion Gap Calculator estimates urinary ammonium excretion from spot urine electrolytes to help classify a normal-anion-gap metabolic acidosis (NAGMA) as renal versus gastrointestinal in origin. It computes UAG = Urine Na⁺ + Urine K⁺ − Urine Cl⁻, with a positive result suggesting impaired renal ammonium excretion (renal tubular acidosis) and a negative result suggesting gastrointestinal bicarbonate loss. The app attributes the original description of this test to Goldstein, American Journal of Nephrology 1986.
The app states a UAG greater than 5 suggests impaired NH4⁺ excretion, pointing toward distal (Type 1) RTA, hypoaldosteronism (Type 4), or proximal (Type 2) RTA with bicarbonaturia.
A UAG below −5 indicates appropriate ammonium excretion, meaning the acidosis is most likely from gastrointestinal bicarbonate loss such as diarrhoea, fistula, or ileostomy.
The app labels values between −5 and 5 as indeterminate and suggests calculating the urine osmolar gap as a surrogate marker instead.
The app's reference notes the UAG is unreliable in the presence of a urinary tract infection (urease-producing organisms), volume depletion, or high levels of unmeasured urine anions.
Spot urine sodium, potassium, and chloride, all in mEq/L.