Therapeutic drug monitoring (TDM) is the clinical practice of measuring specific drugs at designated intervals to maintain a constant concentration in a patient's bloodstream, thereby optimizing individual dosage regimens.

TDM is used mainly for monitoring biological and chemical drugs with narrow therapeutic ranges, drugs with marked pharmacokinetic variability, medications for which target concentrations are difficult to monitor and drugs known to cause therapeutic and adverse effects.
New approaches allow to test drug trough levels using chemical analyzers (instead of HPLC) and recently by new Point-of Care devices producing results within a few minutes e.g. :

- TDM of anti-TNF drugs for IBD and rheumatology

- Monitoring of Immunosuppressive drugs (ISD) to predict transplant rejection and immune-mediated diseases

- Monitoring of Antipsychotic drugs (e.g. clozapine)

- Epilepsy

- Chemotherapies (like 5FU), Immunotherapies (Keytruda)

The goal of TDM is to use appropriate concentrations of difficult-to-manage medications. It allows to optimize clinical outcomes in patients with various clinical situations especially when there is a strong correlation between drug blood levels and efficacy. Furthermore, TDM can improve patient responses to important life-sustaining drugs and to decrease adverse events due to toxicity. Dose optimization is correlated to decreased hospitalizations and cost saving.