NONMEM Users Network Archive

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Linear VS LTBS

From: Indranil Bhattacharya <ibhattacharya>
Date: Thu, 20 Aug 2009 12:06:31 -0400

Hi, while data fitting using NONMEM on a regular PK data set and its log
transformed version I made the following observations

  - PK parameters (thetas) were generally similar between regular and when
using LTBS.
-ETA on CL was similar
-ETA on Vc was different between the two runs.
- Sigma was higher in LTBS (51%) than linear (33%)

Now using LTBS, I would have expected to see the ETAs unchanged or actually
decrease and accordingly I observed that the eta values decreased showing
less BSV. However the %RSE for ETA on VC changed from 40% (linear) to 350%
(LTBS) and further the lower 95% CI bound has a negative number for ETA on
Vc (-0.087).

What would be the explanation behind the above observations regarding
increased %RSE using LTBS and a negative lower bound for ETA on Vc? Can a
negative lower bound in ETA be considered as zero?
Also why would the residual vriability increase when using LTBS?

Please note that the PK is multiexponential (may be this is responsible).

Thanks.

Neil

--
Indranil Bhattacharya

Received on Thu Aug 20 2009 - 12:06:31 EDT

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