Back to the Top
I have a few questions about calculating CrCl that I hope someone can help me
with. Currently my hospital has started a policy of monitoring CrCl by
pharmacy, a good idea, but the problem is we cannot all agree on the 'correct' formula to use. Most of us use ideal body weight in the formula [(140-age)*IBW]/72* SCr (*0.85 for female).
1) Some have used dosing weight in place of IBW in pt's greater than 20% above
IBW. Is this justified?
2) Some have suggested normalizing for BSA. Has this been shown to be more/less
accurate?
3) Some have rounded SCr up to 1 for pt's older than 65. Is this appropiate?
We would like to standardize on one way of calculating CrCl so as not have
conflicting recommendations, as have already happened. Thanks in advance
for any
information.
Chris Humble
Albert Einstein Med Ctr
from: Chris Humble <75407.2305.at.CompuServe.COM>
Back to the Top
> I have a few questions about calculating CrCl that I hope someone can help me
> with. Currently my hospital has started a policy of monitoring CrCl by
> pharmacy,
> a good idea, but the problem is we cannot all agree on the 'correct'
>formula to
> use. Most of us use ideal body weight in the formula [(140-age)*IBW]/72* SCr
> (*0.85 for female).
>
> 1) Some have used dosing weight in place of IBW in pt's greater than 20%
>above
> IBW. Is this justified?
Not really ! The so-called dosing weight is good for dosing
aminoglycosides and similarly distributed drugs. In the case of
creatinine clearance, you need Lean or Ideal Bdy Mass (LBM or IBM) because it
provides a more "accurate" parameter of muscle mass, and hence of the
rate of creatinine production. However, for practical purposes
whether you use LBM or DW what you get is still a ROUGH estimate of
GFR, and hence of renal function. This "ROUGH ESTIMATE" is however
still very useful for the purpose of dose adjustments.
> 2) Some have suggested normalizing for BSA. Has this been shown to be
>more/less
> accurate?
Normalizing for BSA is great for comparison purposes, and for drawing
general guidelines and nomograms. Drug elimination is effected by the
actual (non-normalized) glomerular filtration rrate
and/or tubular secretion rate. For your purposes (and ours)
normalizing is unecessary.
> 3) Some have rounded SCr up to 1 for pt's older than 65. Is this appropiate?
I have seen arguments for it and against it. I happen to use this shortcut
routinely for elderly female pts to get a "quid & dirty" estimate as
opposed to a rough estimate of CLcr.
The more important issue is how to estimate CLcr in elderly pts with low
muscle mass (e.g., muscle
atrophy or emaciated pts). In this case rounding to one is not enough.
Estimating creatinine clearance in these pts has always been problematic. In
these patients the serum creatinine is often deceptively low or
"normal", and the Cockroft equation (Nephron '76; 16: 31-41) yields
inaccurate estimates of renal function. However, recently SANAKA and his
co-workers (Nephron
1996; 73: 137-144) developed an equation that gives CLcr values that
are close to those obtained by direct measurement of creatinine
clearance using 24-hr urine collections.
For males: CLcr = BW (19 Alb + 32) / 100 Scr
For females: CLcr = BW (13 Alb + 29) / 100 Scr
Where: BW = actual body weight in kgs; Alb = serum albumin in grams /
dL; and Scr = serum creatinine in mg/dL.
In conclusion, while it is good to try to obtain as accurate an estimate of
CLcr as
possible, the important issue is what to do with it once you get it.
Let this be our thread for future discussions on this topic....
N. Anaizi, PhD RPh
Univ. of Rochester Med Center
Back to the Top
Chris,
The Cockroft & Gault formula quoted for estimating CrCl can and does
require actual body weight. Ideal body weight may be OK in normal
circumstances but in overweight persons this may be misleading. there is
no harm in normalizing to BSA (1.73 sq m). I know that in diabetics the
CrCl using this formula (using actual weight) has been tested by comparing
it with radio-isotopic method with very good agreement (Sampson & Drury,
1992, Diabetes vol 15 p609-). In my experience this formula also gives
correct estimates of CrCl in HIV subjects when using actual weight when
compared to actual renal clearances of creatinine. You may find that renal
clearance of creatinine overestimates true GFR by some 15-20% in healthy
subjects and in HIV (Noormohamed et al. Br J Clin Pharmacol, (1997) vol 47
(in press). My limited experience with patients with a single kidney also
shows that the formula can be used confidently and compares well with true
GFR measured using 51Chrome-EDTA.
By the way the formula works just as well when using serum creatinine
expressed in micromol/l. In such cases the formula is (140-age[y]) X body
weight [kg] X K/serum creatinine[micromol/l) where K = 1.23 for male and
1.05 for female subjects.. Hope this is useful.
Faruq.
Faruq H Noormohamed
Department of Therapeutics
Chelsea andWestminster Hospital
369 Fulham Road
LONDON
SW10 9NH
Tel +44 (0)181 746 8141
Fax +44 (0)181 746 8887
email f.noormohamed.aaa.cxwms.ac.uk
----------
> From: Chris Humbleby way of David_Bourne <75407.2305.aaa.CompuServe.COM>
> To: Multiple recipients of PharmPK - Sent by <75407.2305.-a-.CompuServe.COM>
> Subject: Creatinine Clearance questions
> Date: Thursday, December 05, 1996 4:32 PM
>
> PharmPK - Discussions about Pharmacokinetics
> Pharmacodynamics and related topics
>
> pharmpk PK/PD Discussion LISTSERV
> I have a few questions about calculating CrCl that I hope someone can
help me
> with. currently my hospital has started a policy of monitoring CrCl by
> pharmacy,
> a good idea, but the problem is we cannot all agree on the 'correct'
formula to
> use. Most of us use ideal body weight in the formula [(140-age)*IBW]/72*
SCr
> (*0.85 for female).
>
> 1) Some have used dosing weight in place of IBW in pt's greater than 20%
above
> IBW. Is this justified?
>
> 2) Some have suggested normalizing for BSA. Has this been shown to be
more/less
> accurate?
>
> 3) Some have rounded SCr up to 1 for pt's older than 65. Is this
appropiate?
>
> We would like to standardize on one way of calculating CrCl so as not
have
> conflicting recommendations, as have already happened. Thanks in advance
> for any
> information.
>
> Chris Humble
> Albert Einstein Med Ctr
>
> from: Chris Humble <75407.2305.-a-.CompuServe.COM>
Back to the Top
There is a good discussion of creatinine clearance formulae in DiPiro's
Pharmacotherapy. The discussion states that the Cockroft-Gault equation was
originally done with ABW not IBW or DBW.
See also Murphy JE, ed. Clinical Pharmacokinetics (pocket reference).
ASHP, 1993, pages xxxv-xxxvii. These latter two references caution on STABLE
vs CHANGING renal function, as well.
Also: Davis GA, Chandler MH. Comparison of creatinine clearance
estimation methods in patients with trauma. American journal of health-system
pharmacy 1996 May 1;53(9):1028-32. This article helps point out some issues
with adjustments, etc., also.
We have a similar problem here. Our decision (for purposes of a
standardized and consistent way to REPORT an estimate of the patient's CrCl)
was to use the Jeliffe formula (114-0.8(Age)/SCr, females 85% of this). This
is reported out in our institutions lab results (it is automatically calculated
every time a SCr is done on a patient). It bypasses the need for weight
(retrieving a height and weight on a patient is not always possible or done!),
and has a disclaimer stating that the estimate is about 9% greater than
actual.
Our Pharmacy order-entry computer (Cerner) automatically calculates using C-G
(based on IBW) when we enter height and weight. The bottom line is for a
screening process: which patients have bad renal function.
My personal approach is to look at the screening data (or just the
patients age) and determine where to go next. I try and look at many factors,
most specifically age, nutritional status (+ or -), and concurrent disease
states. I try to find the factors about the patient that will affect how I
look at their SCr, usually those that I feel who's SCr will NOT reflect their
true renal function (i.e., malnourished patients, patients with liver disease,
etc.) This latter aspect addresses the "rounding up" issue you mentioned. I
do not do it, and I believe my colleagues do not either. (There was a recent
article that addresses this within the past few years that escapes me at this
writing, sorry!) I personally use C-G wehen I know the RF is stable, and then
judge where I feel the patient will trend toward during my monitoring
processes.
Bottom line: use whatever formula to screen and/or initial CrCl estimate, then
use your clinical judgement and experience and talking with colleagues when
faced with difficult (presumed or real) patients.
Marc Semprebon, RPh, MSLS
Dartmouth-Hitchcock Medical Center
Lebanon, NH
Back to the Top
I mis-represented information in my previous note regarding the Jeliffe formula
and our reporting of CrCl. Our comments about the estimated clearance is "As
agroup, this estimate appears to be about 9 ml/min (not %) less than actual
measured clearance. However, in an individual patient, the estimate is within
+/- 20 ml/min 2/3rds of the time, when compared to actual measured value."
Marc Semprebon, RPh, MSLS
Dartmouth-Hitchcock Medical Center
Lebanon, NH
Back to the Top
> I have a few questions about calculating CrCl that I hope someone can help me
> with. currently my hospital has started a policy of monitoring CrCl by
> pharmacy,
> a good idea, but the problem is we cannot all agree on the 'correct'
>formula to
> use. Most of us use ideal body weight in the formula [(140-age)*IBW]/72* SCr
> (*0.85 for female).
>
> 1) Some have used dosing weight in place of IBW in pt's greater than 20%
>above
> IBW. Is this justified?
If you consider the fact that creatinine is produced in the muscle
tissues, rather than adipose tissues, then the use of anything but
IBW is not justified.
>
> 2) Some have suggested normalizing for BSA. Has this been shown to be
>more/less
> accurate?
In general, CrCl is not an accurate nor precise measure of renal
function, but it is the easiest method we have available in clinical
practice. Nonetheless, normalizing CrCl values for a BSA of 1.73 m2
(roughly 70 kg) allows a fair comparison between individuals of
different body habitus. I am assuming that pharmacy is in charge of
calculating CrCl primarily for the purpose of dosing renally
excreted/metabolized drugs. In that regard, one has to be careful in
basing the dosing regimen on normalized CrCl, since the original PK
study for a specific drug may not have normalized subject's CrCl
(many do, however).
>
> 3) Some have rounded SCr up to 1 for pt's older than 65. Is this appropiate?
This does not seem appropriate since I can't figure out why a value
of 1 is chosen. I am assuming it is because it is a round number. In
such patients, a 24-hour collection is necessary.
>
> We would like to standardize on one way of calculating CrCl so as not have
> conflicting recommendations, as have already happened. Thanks in advance
> for any
> information.
>
> Chris Humble
> Albert Einstein Med Ctr
>
> from: Chris Humble <75407.2305.-a-.CompuServe.COM>
Arasb Ateshkadi, Pharm.D.
Assistant Professor
Department of Pharmacy Practice
College of Pharmacy
University of Utah
258 Skaggs Hall
Salt Lake City, UT 84112
TEL: (801) 581-6159
FAX: (801) 585-6160
e-mail: arasb.aaa.deans.pharm.utah.edu
Back to the Top
> If you consider the fact that creatinine is produced in the muscle
> tissues, rather than adipose tissues, then the use of anything but
> IBW is not justified.
Your statement about IBW is a bit too strong. The idea is to predict
creatinine production rate and in part this can be done using a measure
of body size to predict muscle mass. IBW is computed from gender and
height. Total body weight in a person of "normal" body composition has
been the basis for the standard CPR formulae and for this empirical
reason it should be preferred over IBW. In people with abnormal
composition esp. obesity then IBW may be better than TBW when
there is an excess of muscle free mass aka fat. I am not aware of any
reported study that has examined IBW as a predictor of CPR. Does anyone
know of one?
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, Private Bag 92019, Auckland, New Zealand
email:n.holford.aaa.auckland.ac.nz tel:+64(9)373-7599x6730 fax:373-7556
http://www.phm.auckland.ac.nz/Staff/NHolford/nholford.html
Back to the Top
Following the discussion:
>> If you consider the fact that creatinine is produced in the muscle
>> tissues, rather than adipose tissues, then the use of anything but
>> IBW is not justified.
>
>Your statement about IBW is a bit too strong. ...... I am not aware of any
>reported study that has examined IBW as a predictor of CPR. Does anyone
>know of one?
>
Those interested in a study of creatinine clearance estimators and the IBW
v. TBW question may be interested in the following paper:
Hallynck T, Soep HH, Thomis J, Boelaert J, Daneels R, Fillastre JP, De Rosa
F, Rubinstein E, Hatala M, Spousta J and Dettli L, Prediction of creatinine
clearance from serum creatinine concentration based on lean body mass.
Clinical Pharmacology & Therapeutics 30(3): 414-21, 1981.
Joseph Balthasar, PhD
SUNY at Buffalo, Pharmaceutics
Back to the Top
> >Your statement about IBW is a bit too strong. ...... I am not aware of any
> >reported study that has examined IBW as a predictor of CPR. Does anyone
> >know of one?
> >
> Those interested in a study of creatinine clearance estimators and the IBW
> v. TBW question may be interested in the following paper:
>
> Hallynck T, Soep HH, Thomis J, Boelaert J, Daneels R, Fillastre JP, De Rosa
> F, Rubinstein E, Hatala M, Spousta J and Dettli L, Prediction of creatinine
> clearance from serum creatinine concentration based on lean body mass.
> Clinical Pharmacology & Therapeutics 30(3): 414-21, 1981.
Thanks for pointing out this interesting study of Lean Body Mass as a
predictor of CPR. Note however that LBM is NOT the same as IBW. LBM (as
described by Hallynck et al.) is based on the skinfold thickness (at
subscapular and biceps sites) and total body weight.
IBW on the other hand uses gender and height i.e. very different body
properties.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, Private Bag 92019, Auckland, New Zealand
email:n.holford.-a-.auckland.ac.nz tel:+64(9)373-7599x6730 fax:373-7556
http://www.phm.auckland.ac.nz/Staff/NHolford/nholford.html
|
Want to post a follow-up message on this topic?
If this link does not work with your browser send a follow-up message to PharmPK@lists.ucdenver.edu with "Creatinine Clearance questions" as the subject | Support PharmPK by considering my eBooks |
Copyright 1995-2014 David W. A. Bourne (david@boomer.org)