# Re: [NMusers] Why should we avoid using micro rate constants?

From: Saeheum Song <ss.pkpdmodel_at_gmail.com>
Date: Tue, 5 Feb 2019 08:52:39 -0500

The input data are composed of amount as dosing and concentration in
plasma. Concentration is expressed as amount divided by volume of
distribution. The rate constant is movement of either amount or
concentration pending on modelers intention. Whence, in the modeling
fitting, dosing amount needs to be converted into concentration by
introduction of volume of distribution.
With introduction of the volume of distribution (K21),
K12= Q/V1, and
K21=Q/V2
Therefore by introduction of Volume (V1), system can define all other
parameters.

Simply, by model fitting using micro-constant only means we are treating
amount and concentration as same unit. Hence, it should be avoided. You
can still fit model with micro-constant but requires defining volume and
conversion of amount into concentration to do proper modeling,

Regards,

SaeHeum Song,

Independent Consultant,

On Tue, Feb 5, 2019 at 2:05 AM <janet.wade_at_telia.com> wrote:

> Hi All,
>
>
>
> It could also be the statistical model. If you are estimating 4 parameter=
s
> then different parameterisations should be fairly equivalent if a BLOCK(4=
)
> structure is used for both parameterisations. If only the diagonal option
> is used, then this could be why different results/minimisations are
> obtained for different parameterisations.
>
>
>
> Kind regards,
>
> Janet
>
>
>
>
>
> *Janet R Wade, PhD*
>
> *Occams*
> Senior Consultant
>
>
>
>
>
>
>
> *From:* owner-nmusers_at_globomaxnm.com <owner-nmusers_at_globomaxnm.com> *On
> Behalf Of *Leonid Gibiansky
> *Sent:* 04 February 2019 07:30
> *To:* Singla, Sumeet K <sumeet-singla_at_uiowa.edu>
> *Cc:* nmusers_at_globomaxnm.com
> *Subject:* Re: [NMusers] Why should we avoid using micro rate constants?
>
>
>
> It could be just coding error, could you show the control stream?
>
> Thanks
>
> Leonid
>
>
> On Feb 3, 2019, at 12:44 PM, Singla, Sumeet K <sumeet-singla_at_uiowa.edu>
> wrote:
>
> Hello everyone!
>
>
>
> I have a question. I was trying to build a 2-compartment PK model for
> marijuana use in occasional and chronic smokers. Initially, I was using
> providing rate constants K12 and K21 ­in PK block and it resulted in=
poor
> fitting. Then, I later changed to CL,V1, V2 , Q and it resulted in proper
> fitting. I was perplexed as to why I couldn’t get a proper fit by=
providing
> rate constants? I tried to look online but couldn’t find any prop=
er
> explanation about when (or not) we should use micro constants in PK block
> to define our model in NONMEM? Does anyone has any useful insights into
> this?
>
>
>
> Regards,
>
> *Sumeet Singla*
>