- On 16 Aug 2002 at 13:55:53, Rostam Namdari (chista90.-a-.yahoo.com) sent the message

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Dear Colleagues,

Couple of months ago when we were discussing how to

handle BLOQ values for TK/PK analysis almost everyone

shut down the idea of using zero for BLOQs because in

theory there are some drug still in the system,

problems with fitting log of zero and underestimating

the mean. Ignoring them dose not solve the problem as

may result in overestimating the mean value.

1) Did we ever discuss using half of LOQ? What are the

advantages and disadvantages of this approach? I

would appreciate your input.

2) How do we deal with a situation when we have all

the measurements (say data from all three animals) for

the last two time points (say 48 h and 72 h) as BLOQ?

If we assign a value equal to LOQ/2 then we start

getting a flat line and a false long t1/2.

Roger: Can we apply your method (USC*PACK) to

situation where we have background concentrations

(radioactivity data) and not quite LOQs? Please

forgive me if this is an inappropriate question to

begin with.

Rostam - On 16 Aug 2002 at 18:16:57, Roger Jelliffe (jelliffe.aaa.usc.edu) sent the message

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Dear Rostam:

Thanks for your note. I would think it would work fine. Just

subtract out the background, let the results be either positive or

negative, and find the mean and SD. Then you have the SD of a zero. Then,

using the working assay, you can, with the other values (which should also

have the background subtracted out, ba able to determine the SD of any

measurement, using the error polynomial.

On the other hand, if you are using radioactive counts, you should

be able to determine the error of the counts directly also, and use that

for each of your samples, to find the SD of the counts for each sample.

When you get down to the background, you can also determine the SD of the

representative background count.

Does this help? I look forward to hearing from you.

Very best regards,

Roger

Roger W. Jelliffe, M.D. Professor of Medicine,

Division of Geriatric Medicine,

Laboratory of Applied Pharmacokinetics,

USC Keck School of Medicine

2250 Alcazar St, Los Angeles CA 90033, USA

email= jelliffe.at.usc.edu

Our web site= http://www.lapk.org

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