Therapeutic drug monitoring (TDM) is a branch of clinical chemistry and clinical pharmacology that specializes in the measurement of medication levels in blood. Its main focus is on drugs with a narrow therapeutic range, i.e. drugs that can easily be under- or overdosed. TDM aimed at improving patient care by individually adjusting the dose of drugs for which clinical experience or clinical trials have shown it improved outcome in the general or special populations. It can be based on a a priori pharmacogenetic, demographic and clinical information, and/or on the a posteriori measurement of blood concentrations of drugs (pharmacokinetic monitoring) or biological surrogate or end-point markers of effect (pharmacodynamic monitoring). There are numerous variables that influence the interpretation of drug concentration data: time, route and dose of drug given, time of blood sampling, handling and storage conditions, precision and accuracy of the analytical method, validity of pharmacokinetic models and assumptions, co-medications and, last but not least, clinical status of the patient (i.e. disease, renal/hepatic status, biologic tolerance to drug therapy, etc.). Many different professionals (physicians, clinical pharmacists, nurses, medical laboratory scientists, etc.) are involved with the various elements of drug concentration monitoring, which is a truly multidisciplinary process. Because failure to properly carry out any one of the components can severely affect the usefulness of using drug concentrations to optimize therapy, an organized approach to the overall process is critical. A priori therapeutic drug monitoring A priori TDM consists of determining the initial dose regimen to be given to a patient, based on clinical endpoint and on established population pharmacokinetic-pharmacodynamic (PK/PD) relationships. These relationships help to identify sub-populations of patients with different dosage requirements, by utilizing demographic data, clinical findings, clinical chemistry results, and/or, when appropriate, pharmacogenetic characteristics. A posteriori therapeutic drug monitoring The concept of a posteriori TDM corresponds to the usual meaning of TDM in medical practice, which refers to the readjustment of the dosage of a given treatment in response to the measurement of an appropriate marker of drug exposure or effect. TDM encompasses all aspects of this feedback control, namely: it includes pre-analytical, analytical and post-analytical phases, each with the same importance; it is most often based on the specific, accurate, precise and timely determinations of the active and.or toxic forms of drugs in biological samples collected at the appropriate times in the correct containers (PK monitoring), or can employ the measurement of a biological perimeter