Drug Plasma Levels

Table of Contents
  • First of all, 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 takes 4 to 5 drug half-lives.
  • How are you timing the collection of the medication? If you need to take a blood sample 12 hours post-dose, take it 12 hours post dose!
  • Measuring trough levels: To measure a trough (“pre-dose”) sample, you should take the blood sample immediately before the next dose is due. If you need to take a pre-dose blood sample, 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 measurement of the time of the blood sample, and the time of the last dose of medication. This is very important.
  • 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. Some patients respond to lower levels than guideline ranges, 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.