Tlag estimation individually

Phoenix Certara Forums Modelling and Simulation
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Hi,

I’m working on a PopPk model with a dataset that includes plasma samples from two different trials with different study designs. Plasma sampling was taken extensively before cmax in one study, but only three times before cmax in the other. I discovered that including Tlag improved model fit, and that including the impact of study design as a categorical covariate on Tlag had a significant impact, and that including it in the final model reduced OFV. I’m wondering if it’s possible to have the model estimate Tlag for each study individually as two parameters (Tlag s1, Tlag s2) rather than including the impact as a covariate.

Thanks!

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you can use study as a covaraite and then estimate a different Tlag

e.g. tlag = tvlag1 * study ==1 + tvlag2*study==2

this will effectively give you two lags one for each study

e.g tvlag1 = 2 h tvlag2 = 2.5 h

this will be eactly the same as

tlag= tvlag1 (1+studyeffectstudy==2)

wherre tvlag1 = 2 and study effect = 1.25 i.e tvlag 2 for study 2 = 2 *1.25 = 2.5

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Thanks, Sameer, for your answer.

I am trying to apply your suggestion to the PML but I think there is a mistake in my codes

here is PML

test(){
cfMicro(A1, Cl / V, first = (Aa = Ka))
dosepoint(Aa, tlag = Tlag)
C = A1 / V
error(CEps = 1)
observe(CObs = C + CEps * sqrt(1 + C^2 * (CMultStdev/sigma())^2))
stparm(Ka = tvKa * exp(nKa))
stparm(V = tvV * exp(nV))
stparm(Cl = tvCl * exp(nCl))
stparm(tlag = tvlag1 * study==0 + tvlag2*study==1)
stparm(CMultStdev = tvCMultStdev)
fcovariate(study())
fixef(tvKa = c(, 1, ))
fixef(tvV = c(, 134414, ))
fixef(tvCl = c(, 8500, ))
fixef(tvTlag1 = c(, 0.5, ))
fixef(tvTlag2 = c(, 0.5, ))
fixef(tvCMultStdev = c(0, 1, ))
ranef(diag(nV, nCl, nKa, nTlag) = c(1, 1, 1, 1))
}

Can you please guide me how imply your suggestion correctly.
Thanks
}

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