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PharmPK subscribers,
I am trying to determine the effects of age and renal function on the
clearance of a drug. The Jellife and Cockcroft and Gault methods to
determine creatinine clearance from serum creatinine concentrations include
the patients age in the calculations. The inclusion of age in the
creatinine clearance calculation may be confounding the determination of the
effects of age and renal function on drug clearance. Are there any other
accurate methods for calculating creatinine clearance from serum creatinine
concentrations that do not include subject age in the equations?
Thanks,
Ben Suttle
Zeneca Pharmaceuticals
from: SUTTLE Ben
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Because of the age related decline in renal function, I think that it will be
difficult to separate the determination of renal function from a serum
creatinine value in a predictive algorithm without considering age. I would
suggest that 8 or 24 hour urine collections be performed in an experimental
situation to separate the effects of age from renal function. Although the
current gold standard is probably non-radioactive iothalamate clearance using
a continuous infusion with three timed urine collections and mid-point serum
concentrations over 3 hours.
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Creatinine clearance estimates are based on the input/output principle. The
role of age, weight, etc. in the equations are simply estimates of the rate
of endogenous production of creatinine which is the rate in (input). The rate
out (output) is the creatinine clearance, and the ratio of the two is equal to
the serum creatinine.
Creatinine porduction is know to decrease with age. If you leave it out of
the estimation formula, you are neglecting a known variable. Why not measure
drug clearance or elimination directly? I have published a couple of papers
in which we measured drug clearance directly using limited sampling.
Art Harralson, Pharm.D., BCPS
Professor and Vice-Chairman
University of the Pacific
School of Pharmacy
AHARRALSON.aaa.UOP.edu
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