Intestinal hyperabsorption of oxalate in calcium oxalate stone formers: Application of a new test with [C-13(2)]oxalate

In up to one-third of patients with calcium oxalate stones, a hyperoxaluria can be detected. Hyperoxaluria can result from increased endogenous produc tion, from excessive oxalate content of the food, or from intestinal hypera bsorption. For a causal therapy, it is important to discriminate between me tabolic and hyperabsorptive hyperoxaluria. Our new C-13-oxalate test allows this differentiation. Under standardized conditions, 50 mg of disodium sal t of [C-13(2)]oxalic acid was applied. From the amount of labeled oxalate e xcreted in urine as measured by a gas chromatographic-mass spectrometric as say, the intestinal absorption was calculated. Seventy patients with recurr ent calcium oxalate urolithiasis who had no signs of inflammatory bowel dis ease were tested. Their mean intestinal oxalate absorption was 9.2 +/- 5.1% . This was significantly higher than the mean absorption of 50 healthy volu nteers (6.7 +/- 3.9%). There was no difference in oxalate absorption betwee n male (n = 25) and female volunteers. Oxalate absorption correlated with t he oxalate excretion in the 24-h urine (volunteers: r = 0.46, P < 0.01; pat ients: r = 0.62, P < 0.001). Oxalate hyperabsorption was defined as an abso rption exceeding 10%. According to this definition, 34% of the patients had oxalate hyperabsorption; 20% of the volunteers showed a hyperabsorption, t oo. The C-13-oxalate absorption test allows reliable determination of intes tinal oxalate absorption. Because of the use of a stable isotope, this test may be repeated as often as required. It will allow the control of therape utic regimens and also help to unravel genetic influences in stone formatio n.