Measurement of blood urea concentration during haemodialysis is not an accurate method to determine equilibrated post-dialysis urea concentration.
BACKGROUND
The double-pool urea kinetic model requires the measurement of the blood urea concentrations 30 min after haemodialysis (C(t+30)) to calculate equilibrated Kt/V. However, it has been suggested that urea concentrations 30 min before the end of dialysis (C(t-30)) may be representative of C(t+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 C(t+30), we calculated the time (T(c)) before the end of dialysis that blood urea concentrations would be the same as C(t+30). In an additional 33 patients, we measured blood urea concentrations at T(c) and in C(t+30).


RESULTS
We found that C(t-30) was significantly lower than C(t+30) independent of the duration of dialysis. However, there was a significant correlation between Kt/V(t-30) and Kt/V(t+30). The T(c) was 45 min before the end of dialysis. In the additional 33 patients, C(t-45) and C(t+30) were 54+/-17 and 52+/-17 mg/dl (NS), and Kt/V(t-45) and Kt/V(t+30) were 1.27+/-0.21 and 1.29+/-0.18 (NS), respectively. There were significant correlations between C(t-45) and C(t+30) (r=0.96; P<0.001), and between Kt/V(t-45) and Kt/V(t+30) (r=0.82; P<0.001). However, when measurements were analysed individually, 48% of the data points from C(t-45) vs C(t+30), and 42% of the data points from Kt/V(t-45) vs Kt/V(t+30) fell out of the 95% confidence interval of regression line.


CONCLUSIONS
Although C(t-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.
Castro
M C
MC
Division of Nephrology, Hospital das Clínicas, School of Medicine of São Paulo University, 04520-013 São Paulo, Brazil.
Romão
J E
JE
Marcondes
M
M
D000328:Adult; D005260:Female; D006801:Humans; D007676:Kidney Failure, Chronic / Q000097:blood* / Q000628:therapy*; D008297:Male; D008875:Middle Aged; D009994:Osmolar Concentration; D006435:Renal Dialysis; D013997:Time Factors; D014508:Urea / Q000097:blood*
D014508:Urea