Can all drugs benefit from TDM? No, not when one considers the mechanism of action and pharmacology of some drugs. For TDM to be effective and useful, one or more of the following should apply:
- The effective concentration and toxic concentrations are well-defined.
- The pharmacokinetics of the drug are known to be variable.
- The drug is given chronically.
- There is the potential for drug-to-drug interactions.
- The drug exhibits high protein binding.
- The toxicity mimics the indication for the drug; toxicity may not be visible during an exam but will only be revealed with TDM.
- The patient is pregnant, very young, or elderly.
- Compliance or history with the drug is poor.
Unless criteria like these exist, it doesn't make much sense to monitor the drug. Penicillin, for example is never measured in the serum. It has a wide therapeutic window, it is not given chronically, it has minimal toxicity, etc.