Urinary ammonium: paediatric reference values.

Urinary ammonium excretion plays a central role in the renal regulation of acid-based homeostasis, since it represents approximately a half to two-thirds (40-50 mmol/day) of the net acid excreted by the kidneys. In normal anion gap metabolic acidosis, quantifying the ammonium in urine provides information about the origin of the defect in urinary acid excretion. Urinary ammonium measurement is not usually available in clinical laboratories and is estimated by indirect methods, such as urinary anion gap or urinary osmolal gap. These are considered inaccurate when it comes to assessing ammonium concentration (1).