What is the role of the urinary PCO2 test in the workup of hyperchloremic acidosis?

Updated: Sep 03, 2020
  • Author: Sai-Ching Jim Yeung, MD, PhD, FACP; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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The urinary PCO2 during alkaline diuresis reflects the rate of proton secretion in the distal tubule. In an alkaline diuresis induced by infusions of NaHCO3, the intratubular pH is high, and this results in a high rate of proton secretion. Because of the high concentration of bicarbonate in the urine, large quantities of carbonic acid (H2 CO3) form. The carbonic acid dehydrates and forms water and carbon dioxide, thus raising the urinary PCO2.

In healthy individuals undergoing a bicarbonate diuresis, the urine PCO2 should rise to above 70 mm Hg. In patients with secretory defects, ie, the inability to secrete protons in the collecting duct, the urine PCO2 fails to rise above 55 mm Hg. In patients with permeability defects, the CO2 tension rises normally because of the normal proton-pump function and because the H+ gradient does not favor the back-diffusion of protons under conditions of alkaline diuresis. Normal results are also observed in hypoaldosteronism RTA and reversible voltage-dependent defects.

The test is performed by infusing a quantity of NaHCO3 sufficient to raise plasma bicarbonate to greater than 30 mEq/L and urine pH to higher than 7. This can be accomplished with intravenous or oral NaHCO3. With the intravenous route, 7.5% NaHCO3 is infused at a rate of 1-2 mL/min for 2 hours, with hourly samples taken for the duration of the test. The infusion is stopped when the pH from at least 3 urine collections is greater than 7.8. With the oral route, 200 mEq of NaHCO3 is given in divided doses the evening prior to testing, and overnight dehydration is necessary.

An important disadvantage of this test is that false-positive results can occur in persons with concentration defects, because urine bicarbonate concentrations are lower and lead to less carbon dioxide generated. This is significant, because concentration defects are common in persons with dRTA and are a consistent finding in persons with chronic renal failure.

Contraindications to the test are other sodium-retaining states and congestive heart failure.

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