in general, in drug studies you have to study the pharmacokinetics, in order to choose your sample of choice ...
for example there are papers that showed a difference in the concentration of the drug being monitored, in plasma vs whole blood samples.
so, go through the literature carefully, and see whats suitable at your place to work with ; ease of getting the samples, pretreatment procedure, ... etc
all the good luck
Plasma vs Whole Blood for Therapeutic Drug Monitoring of Patients ReceivingFK 506 for Immunosuppression