Nephrol Dial Transplant (2001) 16: 1814-1817
© 2001 European Renal Association-European Dialysis and Transplant Association
Measurement of blood urea concentration during haemodialysis is not an accurate method to determine equilibrated post-dialysis urea concentration
Editor's note Please see also Editorial comment by Kooman et al. pp. 1749–1752.
Division of Nephrology, Hospital das Clínicas of the School of Medicine of São Paulo University, São Paulo, Brazil
Background. The double-pool urea kinetic model requires the measurement of the blood urea concentrations 30 min after haemodialysis (Ct+30) to calculate equilibrated Kt/V. However, it has been suggested that urea concentrations 30 min before the end of dialysis (Ct-30) may be representative of Ct+30. The aim of this study was to validate this suggestion.
Methods. Twenty-two patients underwent haemodialysis for 180, 210, and 240 min. For each patient in each dialysis session, urea exponential decay curve was calculated. Because we measured Ct+30, we calculated the time (Tc) before the end of dialysis that blood urea concentrations would be the same as Ct+30. In an additional 33 patients, we measured blood urea concentrations at Tc and in Ct+30.
Results. We found that Ct-30 was significantly lower than Ct+30 independent of the duration of dialysis. However, there was a significant correlation between Kt/Vt-30 and Kt/Vt+30. The Tc was 45 min before the end of dialysis. In the additional 33 patients, Ct-45 and Ct+30 were 54±17 and 52±17 mg/dl (NS), and Kt/Vt-45 and Kt/Vt+30 were 1.27±0.21 and 1.29±0.18 (NS), respectively. There were significant correlations between Ct-45 and Ct+30 (r=0.96; P<0.001), and between Kt/Vt-45 and Kt/Vt+30 (r=0.82; P<0.001). However, when measurements were analysed individually, 48% of the data points from Ct-45 vs Ct+30, and 42% of the data points from Kt/Vt-45 vs Kt/Vt+30 fell out of the 95% confidence interval of regression line.
Conclusions. Although Ct-45 is useful to estimate Kt/V when assessing mean values, it is not suitable when assessing patients individually. This study demonstrates that the best method to calculate equilibrated Kt/V was a blood sample for urea concentrations 30 min after haemodialysis.
Keywords: equilibrated Kt/V; haemodialysis; intradialytic blood urea concentration; post-dialysis blood urea concentration; urea rebound
Correspondence and offprint requests to: Manuel Carlos Martins Castro, Rua Inhambu, 1069 Apto 72, CEP: 04520-013 São Paulo, Brazil.
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  J. P. Kooman, F. M. van der Sande, and K. M. L. Leunissen Kt/V: finding the tree within the woods Nephrol. Dial. Transplant., September 1, 2001; 16(9): 1749 - 1752. [Full Text] [PDF]
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