Urinary Anion Gap (UAG): Differentiates renal vs. GI causes of non-anion gap metabolic acidosis (NAGMA) by estimating urine ammonium (NH₄⁺) excretion.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Normal Anion Gap Metabolic Acidosis (NAGMA) — after HAGMA has been excluded.
Differentiating Renal Tubular Acidosis (RTA) from extra-renal (GI) causes of NAGMA.
Persistent NAGMA in the absence of obvious GI losses.
Investigating hyperchloraemic acidosis post-saline resuscitation vs. true RTA.
The Clinical Question It Answers
In NAGMA, are the kidneys excreting enough acid (NH₄⁺)? A negative UAG confirms the kidneys are responding correctly — the problem is extra-renal. A positive UAG indicates the kidneys are failing to acidify the urine — pointing to Renal Tubular Acidosis.
Section 2
Formula & Logic
Formula
UAG = Urine Na⁺ + Urine K⁺ − Urine Cl⁻
Interpretation
UAG Negative (< 0)
High NH₄⁺ excretion → Kidneys acidifying normally → Extra-renal loss (Diarrhoea, Fistulae)
UAG Positive (> 0)
Low NH₄⁺ excretion → Kidneys failing to acidify → Renal Tubular Acidosis (RTA) or Hypoaldosteronism
Physiological Basis
NH₄⁺ (ammonium) is the primary way the kidney excretes acid. Because NH₄⁺ is not routinely measured, the UAG uses the principle of electroneutrality: if unmeasured NH₄⁺ is high (in the urine), Cl⁻ will exceed Na⁺ + K⁺, making the UAG negative. Conversely, if NH₄⁺ is absent, the balance shifts positive.
Section 3
Pearls/Pitfalls
RTA Type Differentiaton
Type 1 (Distal RTA)
Positive UAG. Urine pH > 5.5. Typically hypokalaemia. Kidney stones.
Type 2 (Proximal RTA)
Positive UAG during acidosis; can become negative after HCO3 load.
In DKA, ketones (beta-hydroxybutyrate) act as unmeasured anions in urine, making the UAG falsely negative despite impaired NH₄⁺ excretion. In this setting, use the Urine Osmol Gap to estimate NH₄⁺ directly.
Section 4
Next Steps
Action Plan for Positive UAG (RTA)
01
Determine RTA Type: Urine pH, K⁺, and bicarbonate wasting pattern.
02
Type 1 (Distal): Oral Potassium Citrate or Sodium Bicarbonate. Evaluate for Sjögren's, amphotericin toxicity.
Type 4 (Hyperkalaemic): Treat hyperkalaemia. Fludrocortisone if Addison's. Discontinue offending drugs.
Complementary Tools
Anion Gap + Delta-Delta
Winter's Formula
Serum Osmolality Gap
Section 5
Evidence Appraisal
Foundational Paper
The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis.
Batlle DC et al. • N Engl J Med.. 1988;318:594–599. Established the clinical use of the UAG and demonstrated its diagnostic accuracy in RTA vs. GI acidosis.
Section 6
Literature
Development
The Urinary Anion Gap was formally described by Daniel Batlle and colleagues at Northwestern University in 1988. The concept emerged from the fundamental observation that electroneutrality is conserved in urine, just as it is in serum — and that this predictable balance could provide a window into the invisible NH₄⁺ excretion that the kidney performs.
