False positive URR and Kt/V

© 1998 Andrew Lundin, M.D. All rights reserved. Reproduced with permission.

Steinkamp wrote:

> 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 are about

> normal for me. My URR is normally 77 or so and the KT.V is normally l.80

> 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 the

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) and

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) the

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 to

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 to

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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