Curated insights • How it Works • Practical Pearls • Evidence Base
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.
| UAG Negative (< 0) |
| UAG Positive (> 0) |
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.
| Type 1 (Distal RTA) |
| Type 2 (Proximal RTA) |
| Type 4 (Hyperkalaemic RTA) |
| GI Loss (Diarrhoea) |
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.
The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis.
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.
Curated insights • How it Works • Practical Pearls • Evidence Base
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.
| UAG Negative (< 0) |
| UAG Positive (> 0) |
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.
| Type 1 (Distal RTA) |
| Type 2 (Proximal RTA) |
| Type 4 (Hyperkalaemic RTA) |
| GI Loss (Diarrhoea) |
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.
The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis.
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.
