MEASUREMENT OF KINETIC-PARAMETERS FOR UREA IN END-STAGE RENAL-DISEASEPATIENTS USING A 2-COMPARTMENT MODEL

Urea kinetic modeling depends critically on the parameters of the mode l used. When urea is removed during hemodialysis, the kinetic model is quite complex. This experiment describes for the first time the use o f injected stable isotope-labeled urea to define kinetics in ESRD pati ents and compares the magnitude of the two urea compartments in these patients with those of control subjects. Such an experimental approach provides the kinetic data in the most direct manner. A gas chromatogr aph/mass spectrometer-based assay provided quantitation to as little a s 0.2 mol% excess urea. The rapidly equilibrating fraction of the two urea compartments is quantified as 41.2% in ESRD patients and 33.4% in controls (P = 0.24). The rest is in a more slowly equilibrating pool. The urea clearances between these two compartments were near 1 L/min for both sets of subjects. The elimination of urea was due to the meta bolic removal of the N-15 label in both groups of subjects as well as renal elimination in the controls. The nearly threefold larger clearan ce (Cl) of labeled urea removal in controls (Cl = 74.6 mL/min) than in ESRD patients (Cl = 25.4 mL/min; P = 0.015) shows the extent to which renal clearance is more important than metabolism. These direct analy ses of the fractional volumes and inter-compartmental clearances for u rea agree closely with previous measurements during high-efficiency he modialysis and indicate that ESRD patients do not differ significantly from control subjects in these parameters.