Acid Base Learner Series: The Urinary Anion Gap

The Urinary Anion Gap

The cations normally present in urine are Na⁺, K⁺, NH₄⁺, Ca⁺⁺ and Mg⁺⁺.
The anions normally present are Cl^-, HCO₃^-, sulphate, phosphate and some organic anions.
Only Na⁺, K⁺ and Cl^- are commonly measured in urine so the other charged species are the unmeasured anions (UA) and cations (UC).
Because of the requirement for macroscopic electroneutrality, total anion charge always equals total cation charge, so:

Cl- + UA = Na⁺ + K⁺ + UC

Rearranging:

Urinary Anion Gap = ( UA - UC ) = [Na⁺]+ [K⁺] - [Cl^-]

Clinical Use

Key Fact: The urinary anion gap can help to differentiate between GIT and renal causes of a hyperchloraemic metabolic acidosis.

It has been found experimentally that the Urinary Anion Gap (UAG) provides a rough index of urinary ammonium excretion. Ammonium is positively charged so a rise in its urinary concentration (ie increased unmeasured cations) will cause a fall in UAG