Urinary Anion Gap

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.

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.