Please refer to the procedure for Pathologist Approval of Laboratory Tests
The main renal homeostatic response to a metabolic acidosis is an increase in acid excretion as urinary ammonium. In response to a metabolic acidosis, urine ammonium excretion should increase to 200-300 mmol per day if the kidneys are normal. Urine ammonium excretion less than this is consistent with a renal tubular acidosis.
Although urine pH is widely used for this purpose, it is not always a reliable index of acid excretion, and the urine ammonium concentration is a more definitive measurement. Urine osmolar gap and anion gap have been used as surrogates for urine ammonium, but are not accurate enough to be useful clinically.
References
1. Ha, L. Y., W. W. Chiu, et al. (2012). "Direct urine ammonium measurement: time to discard urine anion and osmolar gaps." Ann Clin Biochem 49 (Pt 6): 606-608.
2. Carlisle, E. J., S. M. Donnelly, et al. (1991). "Renal tubular acidosis (RTA): recognize the ammonium defect and pHorget the urine pH." Pediatr Nephrol 5 (2): 242-248.
3. Simpson, D. P. (1971). "Control of hydrogen ion homeostasis and renal acidosis." Medicine (Baltimore) 50 (6): 503-541.