Ammonium is a major urinary buffer that is necessary for the normal excretion of the daily acid load. Its urinary rate of excretion (UNH4) may be increased several fold in the presence of extrarenal metabolic acidosis. Therefore, measurement of UNH4 can provide important clues about causes of metabolic acidosis. Because UNH4 is not commonly measured in clinical laboratories, the urinary anion gap (UAG) was proposed as its surrogate about 4 decades ago, and it is still frequently used for that purpose.