RE: [NMusers] Two compartment model with fixed omega parameters

From: Rik Schoemaker <rik.schoemaker_at_occams.com>
Date: Tue, 27 Jun 2017 11:29:53 +0000

Dear Waroonrat,

I fully support Martin’s suggestions below, but to come back to your original question: the fact that IIVs are set to zero for V2 and Q does not mean the second compartment is ‘gone’. If you would examine your model predictions, inclusion of the second compartment -even without IIV- would result in the characteristic bend in the elimination phase of your compound when viewed on the log-scale. It is quite often that NONMEM FOCE-I cannot estimate IIV for V2 and Q and fixing them to zero can result in perfect predictions of your observed concentration profiles including the two-compartment behaviour. No-one would claim that these parameters are in fact the same for every single subject, just that the data cannot support making them different for your subjects in this case.

Kind regards,

Rik Schoemaker, PhD
Occams Coöperatie U.A.
Malandolaan 10
1187 HE Amstelveen
The Netherlands
www.occams.com<http://www.occams.com>
+31 20 441 6410
rik.schoemaker_at_occams.com<mailto:rik.schoemaker_at_occams.com>

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From: owner-nmusers_at_globomaxnm.com [mailto:owner-nmusers_at_globomaxnm.com] On Behalf Of Martin Bergstrand
Sent: 27 June 2017 11:30
To: Waroonrat Sukarnjanaset <waroonrat_at_hotmail.com>
Cc: nmusers_at_globomaxnm.com
Subject: RE: [NMusers] Two compartment model with fixed omega parameters

Dear Waroonrat,

To know if a model “adequately describe the data” you need to study model diagnostics. Read more for example here: http://onlinelibrary.wiley.com/doi/10.1002/psp4.12161/full

The AIC numbers in your case indicate that the two compartment model is a much better description of your data than the one compartment model (read on AIC here: https://en.wikipedia.org/wiki/Akaike_information_criterion). However, this is only a relative comparison and as pointed out before does not say anything about whether any of the models “adequately describe the data”.

All the best,

Martin Bergstrand, Ph.D.
Senior Consultant
Pharmetheus AB

From: owner-nmusers_at_globomaxnm.com<mailto:owner-nmusers_at_globomaxnm.com> [mailto:owner-nmusers_at_globomaxnm.com<mailto:owner-nmusers_at_globomaxnm.com>] On Behalf Of Waroonrat Sukarnjanaset
Sent: Tuesday, June 27, 2017 10:52 AM
To: nmusers_at_globomaxnm.com<mailto:nmusers_at_globomaxnm.com>
Subject: [NMusers] Two compartment model with fixed omega parameters

Dear NMusers,

I have tried to find an appropriate base model.

I found that two compartment model with fixed Omega of V2 and Omega of Q = 0 (AIC 1860.17) provided smaller AIC than one compartment model (AIC 1921.83) did.

From these findings (no variability on V2 and Q), is it suggesting that one cpt model could adequately describe the data?

I would truly appreciate it if you could give me some suggestions.



Kind regards,



Waroonrat



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Received on Tue Jun 27 2017 - 07:29:53 EDT

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