Therapeutic Drug Monitoring

Abstract

The availability of serum drug concentrations and the definition of therapeutic ranges has increased the utilization of serum drug concentrations. One should keep several principles in mind when using drug concentrations to monitor therapy. The time when a plasma sample is obtained is important, in relationship to both time of dosing and length of time the patient has been taking the drug. In general, a blood sample should be drawn at a time when the quantity of drug entering and leaving the body is in equilibrium. This is referred to as “steady state.” A practical method for determining steady state is to wait a length of time equal to five elimination half-lives of the drug. (The elimination half-life is the time required for the quantity of drug in the body to decrease by 50%.) If the dose is increased or decreased, one should wait an additional five half-lives before sampling.