Determination of protein catabolic rate in patients on chronic intermittent hemodialysis: Urea output measurements compared with dietary protein intake and with calculation of urea generation rate

Abstract

We assessed the agreement between different methods of determining protein catabolic rate (PCR) in hemodialysis patients and the possible influence of postdialysis urea rebound and the length of the interdialytic interval on the PCR determination. Protein catabolic rate derived from measured total urea output was compared with recorded daily protein intake (DPI) and calculated urea generation rate (G), calculated by the interdialytic increase in serum urea and an estimated urea distribution volume using either the Watson equation or 58% of postdialysis body weight, and by single-pool urea kinetic modelling. In 16 patients PCR derived from calculated G by fixed urea distribution volume showed a significant decrease with blood samples obtained 10 minutes after dialysis onward as compared with immediately after dialysis, leading to an approximately 6% decrease at 60 minutes. Protein catabolic rate values derived from blood samples taken 15 to 60 minutes after dialysis were not significantly different. Urea kinetic modelling led to a significant increase in calculated PCR with samples from 5 minutes after dialysis onward and a total increase by 11.5% at 60 minutes. Different methods for determining PCR were compared in 13 clinically stable outpatients treated with conventional hemodialysis on cellulose acetate membrane dialyzers during 1 week. The mean PCR calculated from measured total urea output was 61.3 g/24 hr (range, 43.7 to 83.2 g/24 hr). Assessment of DPI as compared with PCR calculated from measured total urea output was lower by 7.5% (95% confidence intervale [Cl], 1.4 to 17.5). Protein catabolic rate derived from calculated G resulted in a consequent overestimation by 8.5% (95% Cl, 4.5 to 12.5), 21.5% (95% Cl, 16.4 to 26.7), and 21.1% (95% Cl, 17.1 to 25.1) for the Watson formula, 58% of postdialysis body weight, and urea kinetic modelling, respectively. The coefficients of variation for PCR determination were all approximately 6%, with the exception of DPI assessment, which had a coefficient of variation of 10.2%. No influence of the length of the interdialytic interval on protein metabolism was found, except for DPI, which showed a difference of 4.4 g/24 hr (95% Cl, 0.7 to 8.1) between short and long interdialytic intervals. In conclusion, PCR estimates based on different methods of calculating G will lead to overestimation of true PCR derived from measured total urea output and are influenced by postdialysis urea rebound, but not by the length of the interdialytic interval. The results obtained with the use of the Watson formula lead to a small overestimation of PCR with acceptable reproducibility, making this method an attractive option for routine surveillance of PCR. Assessment of DPI shows a higher coefficient of variation and underestimates PCR.