Urine Anion Gap
Calculates the urine anion gap (UAG = urine Na⁺ + K⁺ − Cl⁻) to differentiate renal from non-renal causes of normal anion gap (hyperchloraemic) metabolic acidosis.
Score interpretation
UAG negative (< −20): High urine NH₄⁺ excretion. Appropriate renal acid excretion — acidosis is extra-renal.
→ Consider: GI bicarbonate loss (diarrhoea, fistula, ileostomy), proximal RTA (Type 2). Treat underlying GI cause.
UAG near zero: Indeterminate. May reflect mixed picture or early process.
→ Correlate with clinical context. Measure urine osmolality gap (urine NH₄⁺ × 2 if osmolal gap > 40).
UAG positive (> +20): Impaired urine NH₄⁺ excretion — renal cause of acidosis.
→ Consider: distal RTA (Type 1), Type 4 RTA (hyperkalaemia/hypoaldosteronism), CKD. Nephrology referral for RTA workup.
Interpretation bands for the Urine Anion Gap. Apply clinical judgement and local guidance.
References
- Batlle DC, et al. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. N Engl J Med. 1988;318(10):594–599.
Related
Curated clinical cross-links plus same-class fallbacks.
- Sodium Bicarbonate · Alkali Therapy (Metabolic Acidosis / CKD-Related Acidosis)
- Noradrenaline (Norepinephrine) · Vasopressor (Alpha-1 and Beta-1 Agonist)
- Tranexamic Acid (ICU/Trauma/Surgical) · Antifibrinolytic
- Trimetazidine · Metabolic Anti-Anginal
- Acipimox · Lipid-modifying agent (Nicotinic acid derivative)
- Colesevelam hydrochloride · Bile acid sequestrant
- Hyperkalaemia Management · UK Kidney Association Guidelines 2020; NICE CKD Guidelines
- Rhabdomyolysis · Renal Association 2018; UpToDate 2024
- Hypocalcaemia (Adult) · Society for Endocrinology
- SIADH (Endocrine Perspective) · European Hyponatraemia Guidelines 2014
- Hepatorenal Syndrome · EASL 2018; ICA 2015
- Acute Kidney Injury (AKI) · KDIGO 2012 / NICE AKI 2019
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.