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The Cockroft-Gault equation is widely used in PK for estimation of CrCl
based on SCr and body weight. Amputees, especailly lower limb, have a
decreased muscle mass ( hence a potentially decreased SCr) and an
increased body weight. These two combined could result in an
overestimation of ClCr. One solution would be to use ideal or adjusted
body weight, based on patients height before amputation. Anyone have any
clinical experience with this situation?
Thanks,
Ray Smith Ph.D.
University of Nebraska Medical Center,
Omaha, Nebraska USA.
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The following message was posted to: PharmPK
"Raymond A. Smith (by way of David_Bourne)" wrote:
>
>PharmPK - Discussions about Pharmacokinetics
>Pharmacodynamics and related topics
>
>The Cockroft-Gault equation is widely used in PK for estimation of CrCl
based on SCr and body weight. Amputees, especailly lower limb, have a
decreased muscle mass ( hence a potentially decreased SCr) and an
increased body weight.
I won't offer any empirical clinical experience but will suggest some
theoretical scientific rationale. Amputation and resultant loss of
muscle mass would be expected to decrease creatinine production rate
(CPR) (and all other things being equal a decrease in Scr). C&G and
other empirical formulae for CLcr are actually predicting CPR. With the
assumption that Scr is at steady state then CLcr can be predicted from
CPR/Scr.
Amputation per se would *decrease* body weight but physical inactivity
and other factors may indeed lead to increased adipose mass (which does
not affect CPR) but may lead to *increased* body weight. Some amputees
may increase their body weight by a net increase in muscle mass, and
thus CPR, by athletic training and upper body building.
>These two combined could result in an
>overestimation of ClCr. One solution would be to use ideal or adjusted
body weight, based on patients height before amputation. Anyone have any
clinical experience with this situation?
You should be trying to estimate post-amputation muscle mass.
Pre-amputation ideal body weight predicted from height and sex would be
a starting point and then one might deduct say 15% or so depending on
the extent of the amputation. Upper body builders may need an increase
in predicted CPR because of a net increase in muscle mass.
Nick
Some discussion on this issue can be found here: Holford NHG. The
Quinidine-Digoxin Interaction. N. Eng. J. Med. 302: 864 (1980)
--
Nick Holford, Divn Pharmacology & Clinical Pharmacology University of
Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand
email:n.holford.-at-.auckland.ac.nz
http://www.health.auckland.ac.nz/pharmacology/staff/nholford/
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