False positive URR and Kt/V
© 1998 Andrew Lundin, M.D. All rights
reserved. Reproduced with permission.
- > Can anyone tell me why my URR and KT/V would jump real high?
Just got the
- > lab results and the URR is 93 and KT/V is 3.169. Other results
- > normal for me. My URR is normally 77 or so and the KT.V is normally
- > to 1.90.
As Jim Boag says and you later imply the problem is likely in the way
the post blood is drawn. There are two ways to draw the post dialysis
blood that will give a "more correct" answer.
1. After dialysis is concluded and one is disconnected from the machine,
draw the sample from the needle tubing directly after flushing the blood
to clear the already dialyzed sample out of the tubing. The URR or Kt/V
result will depend on how long one waits after the treatment to draw
blood. Longer will allow more BUN rebound from the tissues into the
blood and the URR will be lower. This is to the patient's advantage if
the facility tries harder to make the URR higher. For this reason many
facilities draw the post BUN in ways that tend to make it lower ie give
a higher URR. One way to do this is:
2. Slow done the blood flow to 50 ml/min (to minimize recirculation)
draw the sample immediately after dialysis, before the rebound can
occur.This method is OK if you get longer dialysis. However, if you are
on shorter hours (<4 hr) and get very high blood flows (>400 ml/min)
second method may be disadvantageous to you.
If this is too complicated, ask questions and I will try to restate.
- Peter Lundin, M.D.
- 26 May 1998
- > What is the correct way to draw the post BUN ?
- > I am a home dialysis patient and we are presently didlayzing 4
1/2 hrs per
- > session.
- > My last ERR was 74% Previous session yielded 64% after taking
a vacation and
- > only having 4 hrs of dialysis while traveling.
- > Joe Constantino
Depends upon what one wants. Methods that make the post BUN lower are
the advantage of those wanting shorter dialysis times. Such methods
would incorrectly draw from the venous blood line, the arterial blood
line during dialysis or as close to the end of dialysis as possible (or
even before dialysis is over).
Methods that tend to make the post BUN higher would favor those who want
to give or get more dialysis treatment. These methods would make sure
avoid dialyzed or recirculated blood and wait some time after the
dialysis treatment to allow for rebound.
I favor drawing the post BUN while disconnected from the machine and
waiting two to five minutes, then drawing from the needle line (either)
after flushing to make sure the blood has not been dialyzed.
- Peter Lundin
- May 27, 1998
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