Drug Plasma Levels

Drug Plasma Levels may be helpful to determine if a patient is adherent to medication, or has the appropriate serum levels of medications for a response.

• First of all, consider is there a clinically useful assay to measure this drug?
  • Not every drug has an assay, in fact a minority of them do.
• The drug needs to be at steady state before the plasma level becomes meaningful.
  • This means it takes 4 to 5 drug half-lives to reach steady state (for most drugs)
• How are you timing the collection of blood sample?
  • If you need to take a blood sample 12 hours post-dose, take it 12 hours post dose!
• 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.
  • Target levels have limitations, and some patients may respond to lower levels than guideline suggestions, while other ranges may change widely amongst laboratories.

• Remember the golden rule: treat the patient, not the level!
  • If a patient is responding to a drug, but the plasma level is low, it does not make sense to increase the dose.