Relationship between urine ammonium ion excretion and urine anion gap in dogs

Acidaemia stimulates renal ammonia production and excretion. This adaptive response allows increased H+ secretion and generation of new bicarbonate. To determine whether a relationship existed between urine ammonium (NH4+) concentration and excretion and urine anion gap (Na+ + K+ - Cl-), ammonium chloride (NH4Cl) was administered per os for 5 days to induce systemic acidaemia in 12 healthy Beagles. During NH4Cl administration, a strong, relationship was apparent between urine NH4+ concentration measured in millimoles per litre and urine anion gap. A significant relationship existed between urine NH4+ excretion measured in millimoles per kilogram of body weight per day and urine anion gap. As urine NH4+ concentration or excretion increased, the urine anion gap became more negative. Before NH4Cl administration (no systemic acidaemia), a weak, but significant relationship was observed between urine NH4+ concentration and urine anion gap. However, a relationship was not evident between urine NH4+ excretion and urine anion gap before NH4Cl administration. Hence, urine anion gap is a reliable index of urine NH4+ concentration and excretion only in dogs with metabolic acidosis. In human beings with distal renal tubular acidosis, NH4+ excretion is inappropriately low and results in a positive urine anion gap. Therefore, as a reliable index of NH4+ excretion, urine anion gap may represent an easy and rapid method to aid in the diagnosis of distal renal tubular acidosis in dogs..