I modelled a pooled plasma concentration-time profile in individuals and then simulated a profile using blood sampling times as per protocol. I then excluded sampling times (up to 3 times) and looked at the ratio of VIF (min sampling/max sampling) at each sampling time. I took a ratio of <1.2 as acceptable (this is the subjective step). I could then predict that if a subject missed, in this case, 3 samples, PK parameters would still be reliably estimated. This approach was based on: Gabrielsson and Weiner (2006) Pharmacokinetic and Pharmacodynamic Data Analysis: Concepts and Applications, 4th ed. Swedish Pharmaceutical Press, Stockholm.
I understand that I could also compare VIF for each parameter of interest.
My question: is this a reasonable approach? Is a ratio of 1.2 acceptable?