Urine Anion Gap Calculator
Urine anion gap (Na + K − Cl) to distinguish renal tubular acidosis from gastrointestinal bicarbonate loss in normal-anion-gap metabolic acidosis, with the urine osmolar gap as a check.
How it works
UAG = urine Na + urine K − urine Cl; < −10 → high NH₄⁺ (GI loss); > +10 → low NH₄⁺ (RTA)
In a normal-anion-gap (hyperchloraemic) metabolic acidosis the two big questions are whether bicarbonate is being lost through the gut or whether the kidney has failed to excrete acid. The kidney excretes acid mainly as ammonium chloride, and ammonium is not routinely measured — but it drags chloride with it, so a urine with lots of ammonium has chloride exceeding sodium plus potassium, giving a negative gap. A positive gap means little ammonium and hence a kidney that cannot acidify properly: renal tubular acidosis. Urine pH and serum potassium then separate the RTA types. The gap is a surrogate that fails whenever other unmeasured anions crowd the urine — ketoacids, toluene metabolites, some antibiotics — where the urine osmolar gap is more reliable.
Worked example
Urine sodium 60, potassium 30, chloride 70: gap +20 — low ammonium excretion; with urine pH 6.2 and serum potassium 3.1 this points to distal (type 1) RTA.
Frequently asked questions
Why is potassium included?
Because the gap is a charge balance: the measured cations (Na⁺, K⁺) minus the measured anion (Cl⁻) reflect the unmeasured cation NH₄⁺ (negative gap) versus unmeasured anions (positive gap).
Does the UAG help in high-anion-gap acidosis?
No — it is only interpretable when the serum anion gap is normal. In ketoacidosis the urinary ketones make the UAG misleading.
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Clinical decision tool for education and for use by trained clinicians alongside judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.