# Modelling time varying covariates Using Monolix suite 2018R2

Dear all,
please let me know if you can help me with this code in mlxtran in monolix,

Now i’m working on oral 1 compartment Pk model for tacrolimus, I’ve covariates that are time- varying,
so according to their manual ,they have to be defiend as regessors in the structural model
I did already every single covariate as a continous/ discrete groups and the code run okey,
My question is:
How can I add different regessors with different model together so i can build the full model???

I’ll post some example from my data and equations
1- I’ve Post-operative days as discret time varying covariate on Clearance
i transformed the PODs into groups and i applied those equations in the mlxtran file:

DESCRIPTION:
The administration is extravascular with a first order absorption (rate constant ka).
The PK model has one compartment (volume V) and a linear elimination (clearance Cl).

[LONGITUDINAL]
input = {ka, V, Cl,POD,beta1,beta2,beta3,beta4}
POD={use=regressor}

EQUATION:
if POD == 1
ClwithPOD = Cl
elseif POD == 2
ClwithPOD = Cl * (1+beta1)
elseif POD == 3
ClwithPOD = Cl * (1+beta2)
elseif POD == 4
ClwithPOD = Cl * (1+beta3)
else
ClwithPOD = Cl * (1+beta4)
end

; PK model definition
Cc = pkmodel(ka, V, Cl=ClwithPOD)

OUTPUT:
output = Cc
table = ClwithPOD

2- I’ve ALT levels as a continuous time-varying covariate on Clearance
I applied those equations in the mlxtran file:
[LONGITUDINAL]
input = {ka, V, Cl,ALT,beta1}
ALT={use=regressor}

EQUATION:
ClwithALT=Cl + (beta1 * ALT)
; PK model definition
Cc = pkmodel(ka, V, Cl=ClwithALT)

OUTPUT:
output = Cc
table=ClwithALT

now both of them and other different covariate were significant as single forward inclusion,
how can i combine all of them together???