What it is
In a normal-anion-gap (hyperchloremic) metabolic acidosis, the kidney’s job is to dump acid as ammonium (NH4+). You can’t measure urine NH4+ on a routine panel, so the urine anion gap (UAG) serves as an indirect window: because NH4+ is excreted with chloride, a high unmeasured-cation load drives chloride up and the calculated gap negative.
Formula
$$ UAG = (U_{Na} + U_{K}) - U_{Cl} $$
All values in mEq/L from a spot urine. The interpretation hinges on the sign:
- Negative UAG → high urinary NH4+ → intact renal acid handling → the acidosis is from GI bicarbonate loss (diarrhoea, ileostomy).
- Positive (or near-zero) UAG → low urinary NH4+ → impaired renal acidification → distal (type 1) RTA.
When to use
Only when you have already established a normal-anion-gap metabolic acidosis. The UAG is meaningless in high-anion-gap acidosis and is the first branch point in working up persistent hyperchloremic acidosis in a child.
Worked example
A toddler with chronic diarrhoea and hyperchloremic acidosis. Spot urine: Na 50, K 25, Cl 90 mEq/L.
$$ UAG = (50 + 25) - 90 = -15\ \text{mEq/L} $$
A negative UAG — consistent with an appropriate renal NH4+ response and GI bicarbonate loss, not RTA. Had the urine been Na 40, K 30, Cl 50, the UAG would be +20, pointing to distal RTA.
Pitfalls
- Volume depletion / avid Na avidity: very low urine Na (< 20) makes the UAG unreliable — the kidney can’t excrete NH4+Cl if there’s no sodium to spare.
- Unmeasured anions (ketones, hippurate from toluene/glue-sniffing, penicillins, bicarbonaturia of proximal RTA) excrete NH4+ with a non-chloride anion, falsely raising the UAG — use the urine osmolal gap instead when suspected.
- Only valid in normal-anion-gap acidosis — confirm the serum gap first.
- It is a qualitative surrogate; it estimates direction of NH4+ excretion, not an absolute value.
Run it: Urine Anion Gap
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.