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: how and when drugs enter the circulation, how long they remain in the blood, and how they are eliminated.

TDM is the clinical assessment of a drug's pharmacokinetic properties. Physicians and pharmacists need to establish that a drug is present at an effective concentration but not at 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.