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Urine Anion Gap

Urine anion gap = urine Na + urine K − urine Cl, a surrogate for urinary ammonium in normal-anion-gap metabolic acidosis. A negative gap suggests GI bicarbonate loss (diarrhoea); a positive gap suggests impaired renal acidification (distal RTA).

Urine sodium (mmol/L or mEq/L)
Urine potassium (mmol/L or mEq/L)
Urine chloride (mmol/L or mEq/L)

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Interpretation

Result	Meaning
Negative	GI loss (appropriate renal NH₄⁺ excretion)
Positive	Renal acidification defect (RTA)

Formula

UAG = U_Na⁺ + U_K⁺ − U_Cl⁻ (mmol/L)

Clinical pearls

Only meaningful in normal-anion-gap (hyperchloraemic) metabolic acidosis.
Unmeasured urinary anions (ketones, hippurate from toluene, D-lactate, penicillins) make the gap positive despite high NH₄⁺; use the urine osmolal gap instead.
Unreliable when urine Na <20 mmol/L (volume depletion).

Examples

urine anion gap una 40 uk 20 ucl 80 uag urine sodium 50 potassium 30 chloride 60 normal gap acidosis, urine Na 35, K 25, Cl 70: RTA or diarrhoea?

FAQ

What does a negative urine anion gap mean?

The kidney is excreting ammonium appropriately, pointing to an extrarenal cause such as diarrhoea ("neGUTive").

When is the urine anion gap unreliable?

With volume depletion (urine Na <20), ketoaciduria or other unmeasured urine anions, and in high-anion-gap acidosis.

Evidence

Batlle DC et al. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. N Engl J Med 1988;318:594–9.

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology