Use the UAG as a surrogate for urinary ammonium excretion in normal-anion-gap (hyperchloremic) metabolic acidosis, where the main unmeasured urinary cation is NH₄⁺. A more negative UAG implies higher ammonium excretion and appropriate renal acid elimination.
Goldstein and colleagues described a strong inverse linear relationship between 24‑hour UAG and urinary ammonium (urinary NH₄⁺ ≈ −0.8 × UAG + 82. Correlation coefficient around 0.97) in patients with metabolic acidosis, supporting use of the UAG when direct ammonium measurement is unavailable.
Batlle and colleagues studied healthy volunteers given ammonium chloride, patients with diarrhoea and patients with impaired distal urinary acidification. Mean UAG was clearly negative in ammonium chloride–loaded volunteers (about −27 mmol/L) and in diarrhoea (about −20 mmol/L), but positive in classic distal renal tubular acidosis, hyperkalaemic distal renal tubular acidosis and selective aldosterone deficiency (means around +23 to +39 mmol/L), with a negative correlation between UAG and urinary ammonium.

Group	Clinical setting	Typical mean UAG (mmol/L)
Healthy volunteers + ammonium chloride	Induced hyperchloremic metabolic acidosis	≈ −27
Diarrhoea	Extrarenal bicarbonate loss	≈ −20
Distal (type 1) RTA	Classic distal renal tubular acidosis	≈ +23
Hyperkalaemic distal RTA	Type 4 renal tubular acidosis	≈ +30
Select aldosterone deficiency	Impaired distal acidification	≈ +39

In practice, a clearly negative UAG (for example ≤−20 mmol/L) in confirmed normal-anion-gap metabolic acidosis favours extrarenal bicarbonate loss (most commonly gastrointestinal losses or urinary diversions) with intact renal acid excretion. A clearly positive UAG (for example ≥+20 mmol/L) suggests impaired renal acid excretion, such as distal (type 1) or hyperkalaemic (type 4) renal tubular acidosis or advanced kidney disease.
The UAG is less reliable when urine sodium is very low, urine flow is high, urine pH is very alkaline (pH >6.5) or substantial unmeasured urinary anions are present (eg ketoacids, D‑lactate, toluene or other toxins). In these settings, the urine osmolal gap or direct urinary ammonium measurement generally correlates better with ammonium excretion.
A negative UAG can still be seen in proximal (type 2) renal tubular acidosis because distal ammonium excretion is preserved. Interpret results alongside urine pH, serum potassium and other features of renal tubular acidosis.

urine anion gap is a physiologic surrogate for urinary ammonium excretion rather than a validated standalone diagnostic test.

Interpretation of urine anion gap in normal-anion-gap metabolic acidosis