Differentiate renal vs GI causes of NAGMA.
UAG = (Na + K) − Cl. Negative UAG (typically < −20): appropriate NH4+ excretion → GI bicarbonate loss (diarrhea). Positive UAG (> 0): impaired renal acid excretion → renal tubular acidosis (RTA). UAG is a surrogate for urine NH4+ (unmeasured cation). Only valid in non-AG metabolic acidosis.