The urinary anion gap (UAG) helps assess the cause of metabolic acidosis by evaluating urinary ammonium excretion. Here’s how to calculate and interpret it quickly:
Calculation
Formula:
UAG = [Na⁺] + [K⁺] – [Cl⁻]
(Concentrations in mmol/L from urine electrolytes)
Steps
Obtain urine electrolyte results (sodium [Na⁺], potassium [K⁺], chloride [Cl⁻]).

Plug values into the formula.

Calculate the result.

Interpretation
Negative UAG (<0 mmol/L): Indicates high urinary ammonium excretion, suggesting a non-renal cause of metabolic acidosis (e.g., diarrhea).

Positive UAG (>0 mmol/L): Suggests low urinary ammonium excretion, pointing to renal causes like renal tubular acidosis (RTA).

Near zero or slightly positive: May require further clinical correlation.

Quick Tips
Context: Use UAG in metabolic acidosis with a normal anion gap.

Key causes:
Negative: GI bicarbonate loss (diarrhea).

Positive: RTA or impaired renal acidification.

Double-check: Ensure urine pH and clinical history align (e.g., urine pH >5.5 in RTA).