Wd. Kloppenburg et al., DETERMINATION OF UREA KINETICS BY ISOTOPE-DILUTION WITH [C-13]UREA AND GAS-CHROMATOGRAPHY ISOTOPE RATIO MASS-SPECTROMETRY (GC-IRMS) ANALYSIS, Clinical science, 93(1), 1997, pp. 73-80
1. Stable urea isotopes can be used to study urea kinetics in humans,
The use of stable urea isotopes far studying urea kinetic parameters i
n humans on a large scale is hampered by the high costs of the labelle
d material, We devised a urea dilution for measurement of the distribu
tion volume, production rate and clearance of urea in healthy subjects
and renal failure patients using the inexpensive single labelled [C-1
3]urea isotope with subsequent analysis by headspace chromatography-is
otope ratio MS (GC-IRMS) of the [C-13]urea enrichment, 2. The method i
nvolves measurement of the molar percentage excess of [C-13]urea in pl
asma samples taken over a 4 h period after an intravenous bolus inject
ion of [C-13]urea, During the sample processing procedure, the plasma
samples together with calibration samples containing a known molar per
centage excess of [C-13]urea are acidified with phosphoric acid to rem
ove endogenous CO2, and are subsequently incubated with urease to conv
ert the urea present in the plasma samples into CO2. The C-13 enrichme
nt of the generated CO2 is analysed by means of GC-IRMS, This method a
llows measurement of the molar percentage excess of [C-13]urea to an a
ccuracy of 0.02%. 3. Reproducibility studies showed that the sample pr
ocessing 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% a
nd between-day CV <1.3%) could be repeated with good reproducibility,
4. In clinical urea kinetic studies in a healthy subject and in a rena
l failure patient without residual renal function, reproducible values
of the distribution volume, production rate and clearance of urea wer
e determined using minimal amounts of [C-13]urea (25-50 mg). 5. becaus
e only low [C-13]urea enrichments are needed in this urea dilution met
hod using GC-IRMS analysis, the costs of urea kinetic studies are redu
ced considerably, especially in patients with renal failure.