Absorptive Hyperoxaluria Leads to an Increased Risk for Urolithiasis or Nephrocalcinosis in Cystic Fibrosis

Background: Hyperoxaluria has been incriminated to account for the increased incidence of urolithiasis or nephrocalcinosis in patients with cystic fibrosis (CF). Hyperoxaluria presumably is caused by fat malabsorption and the absence of such intestinal oxalate-degrading bacteria as Oxalobacter formigenes. To better elucidate its pathophysiological characteristics, we prospectively studied patients with CF by determining these parameters and performing renal ultrasonography twice yearly. Methods: In addition to routine tests in urine (lithogenic and stone-inhibitory substances), the presence of O formigenes was tested in stool, plasma oxalate was measured, and a [ 13 C 2 ]oxalate absorption test was performed in 37 patients with CF aged 5 to 37 years (15 females, 22 males) who were constantly hyperoxaluric before the study. Results: Hyperoxaluria (oxalate, 46 to 141 mg/1.73 m 2 /24 h [0.51 to 1.57 mmol/1.73 m 2 /24 h]; normal, 3.3 mg/1.73 m 2 /24 h [>1.9 mmol/1.73 m 2 /24 h]). Urine calcium oxalate saturation was elevated in 17 patients (5.62 to 28.9 relative units; normal female,