In order to discuss TDM and PGx we need to also introduce the concept of pharmacokinetics. Pharmacokinetics is the study of drug disposition in the body, including how and when drugs enter the circulation, how long they remain in the blood, and how they are ultimately eliminated from the body. TDM is the clinical assessment of a drug's pharmacokinetic properties. Physicians and pharmacists need to establish that a drug is present at a clinically-effective concentration yet does not reach a toxic concentration.
The next few pages will describe some of the factors that determine a drug's disposition in the body. These factors ultimately decide the need for therapeutic drug monitoring.