1. Stable
urea isotopes can be used to study
urea kinetics in humans. The use of stable
urea isotopes for 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 [13C]
urea isotope with subsequent analysis by headspace chromatography-
isotope ratio MS (GC-IRMS) of the [13C]
urea enrichment. 2. The method involves measurement of the molar percentage excess of [13C]
urea in plasma samples taken over a 4 h period after an intravenous bolus injection of [13C]
urea. During the sample processing procedure, the plasma samples together with calibration samples containing a known molar percentage excess of [13C]
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 13C enrichment of the generated CO2 is analysed by means of GC-IRMS. This method allows measurement of the molar percentage excess of [13C]
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 [13C]
urea (25-50 mg). 5. Because only low [13C]
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.