The diagnostic value of the urine to blood carbon dioxide tension gradient for the assessment of distal tubular hydrogen secretion in pediatric patients with renal tubular disorders.

The urine to blood carbon dioxide tension gradient (U-B PCO2) following alkalinization of the urine (pH = 7.8) has been widely used to assess distal tubular hydrogen secretion. The magnitude of the U-B PCO2 is influenced not only by the rate of hydrogen secretion but also by bicarbonate concentration and water abstraction. Simultaneous administration of sodium bicarbonate and dDAVP improve the reliability of the test in healthy children. Children with distal renal tubular acidosis were not able to increase urinary PCO2, while a normal increase was found in patients with proximal renal tubular acidosis and the Fanconi Syndrome. Four out of nine patients with urolithiasis failed to increase urinary PCO2 following NaHCO3 and dDAVP-administration, despite a normal ability to acidify the urine following NH4Cl administration. To assess the effect of acute alterations in urinary concentration on urinary PCO2, the test was carried out in children with central diabetes insipidus. Despite sharp increase in urinary bicarbonate concentration these patients failed to increase urinary PCO2.