Re: Why should we avoid using micro rate constants?
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,
Quoted reply history
On Tue, Feb 5, 2019 at 2:05 AM <[email protected]> wrote:
> Hi All,
>
>
>
> It could also be the statistical model. If you are estimating 4 parameters
> 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:* [email protected] <[email protected]> *On
> Behalf Of *Leonid Gibiansky
> *Sent:* 04 February 2019 07:30
> *To:* Singla, Sumeet K <[email protected]>
> *Cc:* [email protected]
> *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 <[email protected]>
> 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 proper
> 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*
>
> Graduate Student
>
> Dpt. of Pharmaceutics & Translational Therapeutics
>
> College of Pharmacy- University of Iowa
>
>
>
>