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) is useful in the following situations: 1. Drugs with low safety margin-digoxin, anticonvulsants, antiarrhythmics, theophylline, aminoglycosides, lithium, tricyclic antidepressants, vancomycin In case of failure of response without any apparent reason-antimicrobials, To check patient compliance-psychopharmacological agents Monitoring of plasma concentration is of no value for1. Drugs whose response is easily measurable antihypeensives, hypoglycaemics, diuretics, oral anticoagulants(Warfarin), general anesthetics.2. Drugs activated in the body-levodopa.3. 'Hit and run drugs' (whose effect lasts much longer than the drug itself)-reserpine, guanethidine, MAO inhibitors, omeprazole.4. Drugs with irreversible action-organophosphate anticholinesterases, phenoxybenzamine. Ref ; KD Tripathi pharmacology 7th edition (page no; 34)