The blood samples for BUN must not be drawn from the blood lines while you are being dialyzed. Samples should be taken from needle tubing before and five minutes or more after treatment depending on length and efficiency of dialysis. Shorter dialysis times with higher blood flows require a longer wait before blood is drawn, possibly up to 30 minutes – due to a rebound effect – for an accurate measure of your dialyzer efficiency. Use of an appropriate formula for estimating the degree of urea “rebound” may be okay. Please discuss this further with your doctor (or nurse) if you have questions.