A high rate of excretion of
ammonium (NH4+) during chronic
metabolic acidosis should rule out the diagnosis of
distal renal tubular acidosis (RTA). Bearing this in mind, the purpose of this report is to demonstrate that a low urine minus blood PCO2 difference in alkaline urine (U-B PCO2) is a less reliable
indicator of the diagnosis of distal RTA. The patient who is the subject of this report sniffs glue on a chronic, but intermittent basis. He presented with
metabolic acidosis (pH 7.20;
bicarbonate, 10 mmol/L) and an anion gap in plasma of 20 mEq/L. The urine anion gap (-14 mEq/L) and osmolal gap (185 mmol/L [mOsm/kg] H2O) suggested that there was a high, rather than a low, rate of excretion of NH4+. This was confirmed by direct measurement of NH4+ in the urine (101 mumol/min). The high rate of excretion of NH4+ suggested that the
metabolic acidosis was due, in large part, to an abnormally high rate of production of
acid (
hippuric acid, because the rate of excretion of
hippurate was 76 mumol/min). The U-B PCO2 was low (10 mm Hg) on the second hospital day, after the
acidosis was corrected. Potential reasons for the discrepancy between the high rate of excretion of NH4+ and the low U-B PCO2 are discussed.