Urea is a small molecular sized solute (60 Dalton) which is freely filtered across the dialyser membrane. By using a standardised method of urea sampling pre and post dialysis, an assessment of haemodialysis adequacy can be made by measuring urea reduction ratio (URR) > 65%
When sampling the urea level for measuring dialysis adequacy, the blood sample for urea must be the first sample taken (after any blood cultures) and collected in the correct blood bottle. Pre and post urea samples must be taken during the same dialysis treatment.
Urea sampling immidiately at the end of dialysis will result in falsely low measurement of post dialysis urea concentration and an over estimation of dialysis dose. True venous blood urea concentration rises rapidly in the first few minutes after dialysis as the effect of access and cardiopulmonary recirculation dissipate and equilibration of blood urea from poorly dialysed compartements occur. In real time this takes about 30 minutes. It is however, impractical to wait 30 minutes before blood sampling post dialysis and so it is recommended that the “stop-dialysate flow” method is used to collect post dialysis blood urea samples.