Determination of urea kinetics by isotope dilution with [C-13]urea and gas chromatography isotope ratio mass spectrometry (GC-IRMS) analysis

1. Stable urea isotopes can be used to study urea kinetics in humans, The use of stable urea isotopes far studying urea kinetic parameters in humans on a large scale is hampered by the high costs of the labelled material, We devised a urea dilution for measurement of the distribution volume, production rate and clearance of urea in healthy subjects and renal failure patients using the inexpensive single labelled [C-13]urea isotope with subsequent analysis by headspace chromatography-isotope ratio MS (GC-IRMS) of the [C-13]urea enrichment, 2. The method involves measurement of the molar percentage excess of [C-13]urea in plasma samples taken over a 4 h period after an intravenous bolus injection of [C-13]urea, During the sample processing procedure, the plasma samples together with calibration samples containing a known molar percentage excess of [C-13]urea are acidified with phosphoric acid to remove endogenous CO2, and are subsequently incubated with urease to convert the urea present in the plasma samples into CO2. The C-13 enrichment of the generated CO2 is analysed by means of GC-IRMS, This method allows measurement of the molar percentage excess of [C-13]urea to an accuracy of 0.02%. 3. Reproducibility studies showed that the sample processing procedure [within-run coefficient of variation (CV) <2.8% and between-run CV <8.8%] and the GC-IRMS analysis (within-day CV <1.3% and between-day CV <1.3%) could be repeated with good reproducibility, 4. In clinical urea kinetic studies in a healthy subject and in a renal failure patient without residual renal function, reproducible values of the distribution volume, production rate and clearance of urea were determined using minimal amounts of [C-13]urea (25-50 mg). 5. because only low [C-13]urea enrichments are needed in this urea dilution method using GC-IRMS analysis, the costs of urea kinetic studies are reduced considerably, especially in patients with renal failure.