Therapeutic  Drug  Monitoring  (TDM)  involves measurement of plasma concentrations of drugs to find whether the drug levels are within the therapeutic range or not. For TDM to be clinically useful, the following criteria should be fulfilled:
Therapeutic Drug Monitoring (TDM)

TDM is a process by which the dose of a drug is adjusted according to its plasma concentration.
For performing TDM, there should be good relation between drug concentration and response. Note that there may not be good relation between dose and plasma concentration.
It is done for drugs having wide variation in pharmacokinetics (absorption, metabolism or excretion), both intra as well as inter- individual.
It is done for the drugs having low therapeutic index like theophylline, lithium, antiepileptics, immuno-modulators and antiarrhythmics etc.
TDM is done for those drugs whose eﬀect cannot be easily measured (like eﬀect of  antihypertensive drugs can be easily measured by monitoring BP, so TDM is not used).
TDM is not done for the drugs which are activated in the body or produce active metabolites,  when pharmacological tolerance is suspected and when there is poor relation between drug concentration and eﬀect.
There should be good relationship between plasma concentration and drug dosage
The relationship between plasma drug concentration and therapeutic response  and/or toxicity should be poor
When pharmacodynamic tolerance is suspected
When the clinical response cannot be easily monitored