Urine Anion Gap

Name: Urine Anion Gap calculator — MedScore
Creator: Nabil Alzein

Spot-urine Na⁺ + K⁺ − Cl⁻, a rough index of urinary ammonium in hyperchloraemic metabolic acidosis

Urine sodium (mmol/L)

Urine potassium (mmol/L)

Urine chloride (mmol/L)

Enter urine sodium, potassium, and chloride to calculate

About this score

When to use

In hyperchloraemic metabolic acidosis, the urine anion gap (Na⁺ + K⁺ − Cl⁻) is a rough index of urinary ammonium excretion. ⁦[1]⁩

Pearls and pitfalls

• It was negative in diarrhoea (−20 ± 5.7 mmol/L) and in normal subjects given ammonium chloride, but positive in patients with impaired distal urinary acidification. ⁦[1]⁩

• In diarrhoea the gap stayed negative even though urine pH was above 5.3, so urine pH alone can mislead. ⁦[1]⁩

Summarised from the numbered sources under Evidence. Check the source before relying on a statement.

Evidence

[1] Original publication

Batlle DC, Hizon M, Cohen E, Gutterman C, Gupta R. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. UAG = Na⁺ + K⁺ − Cl⁻ as a rough index of urinary ammonium: negative in diarrhoea (−20 ± 5.7 mmol/L) and in normal subjects given NH₄Cl (−27 ± 9.8), positive in all patients with altered distal urinary acidification (distal RTA, hyperkalaemic distal RTA, selective aldosterone deficiency).

New England Journal of Medicine, 1988

Open source

Last reviewed: 2026-10

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MedScore is an educational and clinical decision-support tool. It does not provide a diagnosis, a treatment, or a drug-dosing recommendation, and it does not replace clinical judgement or local guidelines. Confirm every result against the original source before acting on it. The treating clinician remains responsible for all decisions.