Urine Anion Gap
Useful in nonanion gap, hyperchloremic metabolic acidosis.
UAG = Urine anions (Na+ + K+) - Urine cations (Cl-).
NH4+ is the major unmeasured urinary cation, a negative UAG reflects high NH4+ excretion, which is an appropriate response to a metabolic acidosis.
UAG estimates amount of NH4+
** Check UAG if there is NAGMA**
UAG : positive = RTA, indicates low NH4+ excretion, which, in the face of a metabolic acidosis, suggests a RTA.
UAG : neg = GI loss HCO3.
UAG is a way to distinguish between diarrhea and RTA as the cause of normal AG metabolic acidosis.
When acid is excreted from the kidney, it goes out as NH4Cl-. Acid excretion from the kidney goes out with chloride, so the urine chloride goes up. If the urine chloride goes up, then the number UAG is negative. Diarrhea causes a negative UAG, because the kidney can excrete acid and the net UAG is negative. In metabolic acidosis, a negative UAG mean the kidney is working fine.
However, if you cannot excrete acid from the kidney, the urine chloride goes down. This give a positive number UAG. In RTA, you cannot excrete acid from the kidney. The urine chloride will be low, and the UAG will be positive.