First of all, consider is there a clinically useful assay to measure this drug?
The drug needs to be at steady state before the plasma level becomes meaningful.
How are you timing the collection of blood sample?
To measure a trough level (“pre-dose”) level, you should take the blood sample immediately before the next dose is due.
If you need to take a pre-dose level, never withhold the next dose for more than 1 to 2 hours; the longer you withhold it, the likelier it will give you a false low reading. You will end up giving an inappropriate dose increase.
The longer the half-life a medication is, the less important it is when you time the sample of the drug
Basic good practice requires recording of the time of the blood sample, and the time of the last dose of medication.
If you are using a plasma level to check adherence, keep in mind that a negative plasma level only means that a drug has not been taken in the past several days.