# Conditional Dosing Pharmacometric Example

##### Chris Rackauckas

In this example we will show how to model a conditional dosing using the DiscreteCallbacks. The problem is as follows. The patient has a drug A(t) in their system. The concentration of the drug is given as C(t)=A(t)/V for some volume constant V. At t=4, the patient goes to the clinic and is checked. If the concentration of the drug in their body is below 4, then they will receive a new dose.

For our model, we will use the simple decay equation. We will write this in the in-place form to make it easy to extend to more complicated examples:

using DifferentialEquations
function f(du,u,p,t)
du[1] = -u[1]
end
u0 = [10.0]
const V = 1
prob = ODEProblem(f,u0,(0.0,10.0))

ODEProblem with uType Array{Float64,1} and tType Float64. In-place: true
timespan: (0.0, 10.0)
u0: [10.0]


Let's see what the solution looks like without any events.

sol = solve(prob,Tsit5())
using Plots; gr()
plot(sol)


We see that at time t=4, the patient should receive a dose. Let's code up that event. We need to check at t=4 if the concentration u[1]/4 is <4, and if so, add 10 to u[1]. We do this with the following:

condition(u,t,integrator) = t==4 && u[1]/V<4
affect!(integrator) = integrator.u[1] += 10
cb = DiscreteCallback(condition,affect!)

DiffEqBase.DiscreteCallback{typeof(Main.WeaveSandBox18.condition),typeof(Ma
in.WeaveSandBox18.affect!),typeof(DiffEqBase.INITIALIZE_DEFAULT)}(Main.Weav
eSandBox18.condition, Main.WeaveSandBox18.affect!, DiffEqBase.INITIALIZE_DE
FAULT, Bool[true, true])


Now we will give this callback to the solver, and tell it to stop at t=4 so that way the condition can be checked:

sol = solve(prob,Tsit5(),tstops=[4.0],callback=cb)
using Plots; gr()
plot(sol)


Let's show that it actually added 10 instead of setting the value to 10. We could have set the value using affect!(integrator) = integrator.u[1] = 10

println(sol(4.00000))

[0.183164]

println(sol(4.000000000001))

[10.1832]


Now let's model a patient whose decay rate for the drug is lower:

function f(du,u,p,t)
du[1] = -u[1]/6
end
u0 = [10.0]
const V = 1
prob = ODEProblem(f,u0,(0.0,10.0))

ODEProblem with uType Array{Float64,1} and tType Float64. In-place: true
timespan: (0.0, 10.0)
u0: [10.0]

sol = solve(prob,Tsit5())
using Plots; gr()
plot(sol)


Under the same criteria, with the same event, this patient will not receive a second dose:

sol = solve(prob,Tsit5(),tstops=[4.0],callback=cb)
using Plots; gr()
plot(sol)


## Appendix

This tutorial is part of the DiffEqTutorials.jl repository, found at: https://github.com/JuliaDiffEq/DiffEqTutorials.jl

To locally run this tutorial, do the following commands:

using DiffEqTutorials
DiffEqTutorials.weave_file("models","02-conditional_dosing.jmd")

Computer Information:

Julia Version 1.1.1
Commit 55e36cc308 (2019-05-16 04:10 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-6.0.1 (ORCJIT, ivybridge)


Package Information:

Status ~/.julia/environments/v1.1/Project.toml
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