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I'm going to do my study for PHARMACOKINETICS OF VANCOMYCIN IN CRITICALLY ILL PATIENTS.
The primary objective is to characterize vancomycin pharmacokinetic parameters in
critically ill patients who has or suspect for
gram-positive bacterial infections and to obtain the guidance for vancomycin dosing
regimens in critically ill patients.
The sample size is determined using the formula for estimating population mean in one
group of descriptive studies.
However, the PK parameters required are the value of the volume of distribution and
How can I calculate sample size because I have SD and mean both Vd and CL from my
Senior Pharmacy Practice Specialist
Burungrad International, Bangkok Thailand.
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The following message was posted to: PharmPK
> I'm going to do my study for PHARMACOKINETICS OF VANCOMYCIN IN CRITICALLY ILL
> The primary objective is to characterize vancomycin pharmacokinetic parameters in
> critically ill patients who has or suspect for
> gram-positive bacterial infections and to obtain the guidance for vancomycin dosing
> regimens in critically ill patients.
Pharmacokinetics is a descriptive and predictive science. Your stated
objectives are relating to learning not confirming questions. Therefore
hypothesis tests to test null hypotheses are not really relevant.
Answers to PK questions depend as much on the number and timing of blood
samples than they do on the number of subjects.
If you have a good question to ask about vancomycin PK e.g. how quickly
does vancomycin clearance change during the care of critically ill
patients? then you need to use simulation or optimal design based
methods to design the study. You will need to learn about using mixed
effect PK software and how to construct models that describe the data
you plan to collect.
Once you have learned how to do this then I think you will understand
why PK studies are not designed using statistical ideas such as "sample
size is determined using the formula for estimating population mean in
one group of descriptive studies".
Note that PubMed lists 7 studies matching "vancomycin pharmacokinetics
critically ill". An important part of your planning should be to read
these studies and understand more clearly what unanswered question you
want to try to answer.
Nick Holford, Professor Clinical Pharmacology
Dept Pharmacology & Clinical Pharmacology
University of Auckland,85 Park Rd,Private Bag 92019,Auckland,New Zealand
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I pretty much agree with Nick, especially with his comment regarding - what new question
are you seeking to answer with this study? While the purpose of PK studies is to generate
good quality PK information, the challenge is in defining 'good quality'. You can plan a
PK study (with respect to sample size, sampling times and analysis methods), say, to
estimate the mean CL and/or V with a 20% precision (SEM). FDA requires this criteria in
accepting the design of the PK studies in pediatrics, where applicable. Dr Yaning Wang et
al from the FDA are in the process of publishing a methods paper on this topic.
Joga Gobburu, PhD, FCP, MBA
Schools of Pharmacy, Medicine
University of Maryland
Room N407, 20 N.Pine St, Baltimore, MD 21201
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