As mentioned, not all drugs are good candidates for TDM. Clinical labs do not routinely measure most drugs. The main reason is that most drugs are not given chronically, or their therapeutic window is so large that dosing is not that difficult. Below are some other factors that would make TDM impractical:
- For drugs that act intracellularly or that need to be converted to active forms (like AZT). The serum level wouldn't reflect the active intracellular drug level.
- For drugs in which the effects last much longer than the serum concentrations of the drugs, examples include antineoplastics (cancer chemotherapies) and warfarin.
- For narcotic pain medications in which continued use leads to tolerance such that the levels needed for pain relief in one person would be toxic to another person.
