Measurement of blood urea concentration during haemodialysis is not an accurate method to determine equilibrated post-dialysis urea concentration

Abstract

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.