The urine-blood PCO2 gradient as a diagnostic index of H+-ATPase defect distal renal tubular acidosis

The urine-blood PCO2 gradient as a diagnostic index of H+-ATPase defect distal renal tubular acidosis. Urine pH during acidemia and urine PCO2 upon alkalization both may be useful to indicate H+ secretion from collecting ducts. The urine anion gap has been used to detect urinary NH4+ for differential diagnosis of hyperchloremic metabolic acidosis. We have previously demonstrated that the lack of normal H+-ATPase may underlie secretory defect distal renal tubular acidosis (dRTA). In this study we evaluated the diagnostic value of the urine-blood (U-B) PCO2 in H+-ATPase defect dRTA, and compared it with that of urine pH and urine anion gap during acidemia. In H+-ATPase defect dRTA, the diagnostic values of three urinary parameters were evaluated: (1) urine pH measured after acid (NH4Cl) loading; (2) urine-to-blood carbon dioxide tension gradient (U-B PCO2) during alkali (NaHCO3) loading; and (3) urine anion gap during acidemia. Seventeen patients were diagnosed as having H+-ATPase defect dRTA based on reduced urinary NH4+ and an absolute decrease in H+-ATPase immunostaining in intercalated cells on renal biopsy. Eight patients with non-dRTA renal disease served as control patients. Upon NaHCO3 loading, U-B PCO2 was ≤30 mm Hg in all 17 dRTA patients and >30 mm Hg in all 8 control patients. With NH4Cl loading, urine pH was >5.4 in 15 of 17 dRTA patients and ≤5.4 in 7 of 8 control patients, and the urine anion gap was >5 mmol/L in 13 of 17 dRTA patients and≤5 mmol/L in 6 of 8 control patients. Therefore, the sensitivity and specificity of U-B PCO2≤30 mm Hg during NaHCO3 loading were both 100%, whereas those of urine pH >5.4 or urine anion gap >5 mmol/L during NH4Cl loading were below 90%. In control patients, the U-B PCO2 was found to be well correlated with the urinary NH4+ (r = 0.79, P < 0.05). The U-B PCO2 during NaHCO3 loading is an excellent diagnostic index of H+-ATPase defect dRTA.