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"Paul Hutson (by way of David Bourne)" wrote:
> Are you aware of any clinical papers in which the rounding-up of
> serum creatinine to 1 mg/dl (or say, 90 uM) is proposed or validated?
I'm not sure if you are asking specifically about using 1 mg/dL (88.4
uM) or the use of other round up concentrations. Carl Kirkpatrick et
al. (2001) has reported on the use of 0.68 mg/dL (60 uM) as a round
up concentration in a large series of patients treated with
gentamicin.
One interpretation of why this empirical round up method works is to
assume that patients with Scr less than the round up concentration
have "normal" Clcr (see below). Presumably they have a low Scr
because of decreased creatinine production that is not recognized by
age, weight and sex e.g. due to cachexia from chronic illness.
In that case the C&G formula is being (ab)used to predict lower CLcr
in older patients and those with lower weight (reasonable) and in
woman (not supported by data as far as I know). The relationship
between age and decreased creatinine production predicted by C&G
presumably correlates with the relationship between age and decreased
renal function. Note that there is no a priori reason to expect that
the shape or parameters of these two relationships would be the same
for age. It might be reasonable to suppose that the decrease in renal
function and creatinine production rate decrease with the same
allometric relationship with weight based on the principle that
functional properties tend to scale according to the 3/4 power model.
In essence the round up method assumes that a 40 year old 72 kg man
would have a Clcr of 147 ml/min [(140-40)/0.68] and that Clr
decreases linearly with age such that CLcr would be 117 mL/min at 60
y and 88 mL/min at 80 y. Corresponding values in a women would be
125, 100 and 75 mL/min. Actual values would be lower with age and in
women because their weights would typically be less than 72 kg.
Kirkpatrick CMJ, Duffull SB, Begg EJ. Pharmacokinetics of gentamicin
in 957 patients with varying renal function dosed once daily. British
Journal of Clinical Pharmacology 2001;47:637-643
--
Nick Holford, Divn Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand
email:n.holford.aaa.auckland.ac.nz
http://www.health.auckland.ac.nz/pharmacology/staff/nholford/
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Copyright 1995-2010 David W. A. Bourne (david@boomer.org)