Dialysis adequacy is traditionally measured as either Kt/V (most popular in outpatient dialysis units) or urea reduction ratio (URR, most popular with acute dialysis/inpatient dialysis units). Both methods use the degree to which urea is cleared--by comparing the urea concentration before and after dialysis--as the central means by which adequacy is determined. Regardless of which method is chosen, the means by which these values are obtained is vitally important.
The "pre-dialysis" BUN should be drawn before dialysis is actually begun. It's okay to hook the patient up to the machine, but the blood sample should be drawn before the actual treatment is begun.
The "post-dialysis" BUN must be drawn in a manner described in the KDOQI Guidelines. The blood is drawn from the arterial line at the end of the treatment; one of the main risks here is that there is always some degree of access recirculation, in which some percentage of just-dialyzed blood makes it back into the arterial line. This will have the effect of artificially lowering the post-dialysis BUN, and can lead to falsely high URR or Kt/V readings. To avoid access circulation problems, the dialysis nurse should slow down the blood flow through the dialyzer to a rate at which access recirculation is negligible. Generally, a blood flow of 100 mL/min for 15 seconds is enough to clear the blood occupying the "dead space" of access plus tubing.