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For a single, extra vascular dose, the plasma concentration of a drug at time zero is theoretically also zero. However, analytical procedures tend to report such "zero data" as below limit of detection (LOD) or below limit of quantification (LOQ). For data entry into PK modeling software, LOD or LOQ are non-valid entry terms and have to be replaced by an integer. Here is where it gets tricky, theoretically at t = 0, one would enter thus 0. However, our more purist statistical colleagues have another opinion and treat LOD and LOQ differently, where it is commonly replaced by a concentration value derived from the method SD. ForIs there any formal guidance on this?
Dr Guus Duchateau
science leader Bioavailability & ADME
Unilever R&D Vlaardingen
PO Box 114, 3130 AC Vlaardingen
The Netherlands
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Dear all,
Can any body briefly explain the standard BQL set of rules tobe followed in PK and PD modeling which are routinely followed.
With regards,
Raghav
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The following message was posted to: PharmPK
Dear Guus,
You wrote:
> For a single, extra vascular dose, the plasma concentration of a drug at
> time zero is theoretically also zero.
Yes. And for this reason, on my opinion, the plasma concentration at time
zero should not be regarded as an observation, since it does not contain any
information, i.e., the model parameters are not dependent on the value of
this observation, since the predicted value is zero for any value of the
model parameters.
> but for t = 0, I would insist on entering simply zero.
Since this observation does not contain information, I would advise to
exclude this observation from the data. As an additional advantage, it
avoids the dilemma of LOQ values (which has been discussed many times in
PharmPK!).
> Is there any formal guidance on this?
I also would like to know.
best regards,
Hans Proost
Johannes H. Proost
Dept. of Pharmacokinetics, Toxicology and Targeting
University Centre for Pharmacy
Antonius Deusinglaan 1
9713 AV Groningen, The Netherlands
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