Bhaskaran S, Tobe S, Saiphoo C, Manuel A
Calculating Kt/V using intradialytic urea values taken 30
minutes before the end of dialysis
43rd Annual ASAIO Conference, Atlanta
ASAIO J
(Apr) 43:75 1997
One of the predictions of the regional blood flow model is, that post-
dialysis urea
rebound can be predicted by a
simple formula:
Kt/V-dp = Kt/V-sp - 0.60 x (Kt/V-sp)/t + 0.03
James Tattersall, based on empiric observations from the standard 2-pool
model,
independently came up with
another relationship: namely, that Kt/V-dp = Kt/V-sp measured exactly
35 minutes
prior to the end of dialysis. Actually, the two are algebraically closely
linked. For
example, if the Kt/V-sp is 1.2 over 4 hours, then K/V is 0.3/hr, or 0.15 per
30 min, and
rebound, or delta Kt/V will be 0.15 as predicted by the Tattersall method.
With the rate
equation, delta Kt/V is 0.6 x 0.3 - 0.03, or 0.15. So both equations will
give an
equilibrated Kt/V of 1.2 minus 0.15, or 1.05. Thus, it is not surprising
that both the
Tattersall method and the Daugirdas/Schneditz rate equation give very similar
results.
However, the rate equation should theoretically have an increased error, as
its prediction
of equilibrated post-BUN depends on both the post-BUN and the pre-BUN,
whereas the
Tattersall method relies on the post-minus-30 min BUN only.
In this paper, Bhaskaran et al measured post+30 min BUN (which they took to
be
an equilibrated post concentration) and compared estimates of this as
predicted by the
Tattersall method (using a post-minus-30 min BUN sample) and by the rate
equation. They
found that the "equilibrated" post-BUN was better predicted by the Tattersall
method (r =
0.93) than by the rate equation (r = 0.79). The abstract is difficult to
analyze, as key
data are not given.
Comment: The data are consistent with statistical theory; i.e., that
it is easier
to predict a BUN concentration 30 min after dialysis with a single-BUN 30 or
35 min prior
to the end of dialysis than with a formula based on both the pre and post
BUN, as with the
latter, errors in two measurements are involved. Having said this,
regression
coefficients are a poor way to compare results, and a mean percent error or
absolute value
percent error would give a better comparison. Also, the n of cases is not
known. One
problem is, that with either method, great care must be taken in how either
the post-30
min sample (Tattersall method) or post sample (rate equation) are drawn. A
slow flow
method must be used to eliminate access recirculation; otherwise, either of
the methods
will give a falsely elevated estimate of equilibrated post BUN. It may be
more practical,
however, in a busy dialysis unit, to draw the post-sample at the end of
dialysis, than to draw a sample at
precisely 30 or 35 minutes prior to the end of dialysis, and the Tattersall
technique
makes it fairly easy to "cheat"; e.g., delay the time at which the post
specimen is drawn.
Both methods are fairly accurate, and the good results with the blood sided
methods
suggest that alternative dialysate quantification methods to estimate
equilibrated Kt/V
may not be as useful as previously thought. (John T. Daugirdas, M.D.,
University of
Illinois at Chicago)
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43rd Annual ASAIO Conference, Atlanta
Basic hemodialysis :
Adequacy, prescription, urea kinetics