The urine0blood PCO2 gradient after NaHCO2 loading is specific for H+-ATPase defect distal renal tubular acidosis. The authors are from Seoul National University, Bundang Hospital, Hanyang University and Catholic University in Seoul, Korea.
Background: Urine PCO2 with alkalinization is a measure of the capacity of the proton pump to maximally secrete H+ into urine. The inability to raise urine PCO2 normally during sodium bicarbonate loading is a sensitive index of decreased distal urinary acidification.
Patient selection: hyperchloremic metabolic acidosis
NaHCO3 loading test: 2.75% NaHCO3 solution infused IV at a rate of 4 mL per kg per hour.
Testing:
(1) Urine and blood samples are collected at 2 hour intervals.
(2) The endpoint is a plasma bicarbonate concentration 26 mmol/L.
(3) The urine pH is raised to 7.5.
(4) PCO2 is measured in blood and urine using a blood gas analyzer (Nova).
urine-blood PCO2 gradient in mm Hg =
= (urine PCO2 in mm Hg) - (plasma PCO2 in mm Hg)
Interpretation:
• A gradient <= 30 mm Hg is 100% sensitive and 100% specific for H+-ATPase defect distal RTA.
• The gradient is well-correlated with the urinary ammonia.
