Board Review Course & Update

Explanation

The patient has a classic clinical presentation of distal renal tubular acidosis (RTA), with a history of Sjogren's syndrome, multiple calcium phosphate kidney stones with nephrocalcinosis, and a non–anion gap metabolic acidosis. In the setting of a systemic metabolic acidosis and normal distal tubular acidification, ammonia excretion should increase and can be directly measured. In the absence of direct measurement, estimation of inadequate ammonia excretion using an inappropriately alkaline urine pH, positive urinary anion gap—([UNa⁺] + [UK⁺]) – [UCl⁻], where UNa⁺ is urine sodium and UCl⁻ is urine chloride—and low (<40 mOsm/kg) urinary osmolar gap has been used to confirm the presence of distal RTA. Although none is as reliable as the direct ammonia measurement, the urinary pH has the most variability. The urinary anion gap is unreliable to differentiate from proximal RTA and can also be affected by the presence of unmeasured anions, such as ketones or hippurate. Because ammonia is an unmeasured osmole, its presence in the urine will increase the urine osmolar gap to above 150 mOsm/kg during metabolic acidosis, representing appropriate distal acidification. Thus, a low urine osmolar gap is the answer.