Pharmacokinetics is currently defined as the study of the time course of drug absorption, distribution, metabolism, and excretion. Clinical pharmacokinetics is the application of pharmacokinetic principles to the safe and effective therapeutic management of drugs in an individual patient. Primary goals of clinical pharmacokinetics include enhancing efficacy and decreasing toxicity of a patient’s drug therapy. The development of strong correlations between drug concentrations and their pharmacologic responses has enabled clinicians to apply pharmacokinetic principles to actual patient situations.

A drug’s effect is often related to its concentration at the site of action, so it would be useful to monitor this concentration. Receptor sites of drugs are generally inaccessible to our observations or are widely distributed in the body, and therefore direct measurement of drug concentrations at these sites is not practical. For example, the receptor sites for digoxin are thought to be within the myocardium. Obviously we cannot directly sample drug concentration in this tissue. However, we can measure drug concentration in the blood or plasma, urine, saliva, and other easily sampled fluids.

Kinetic homogeneity describes the predictable relationship between plasma drug concentration and concentration at the receptor site where a given drug produces its therapeutic effect. Changes in the plasma drug concentration reflect changes in drug concentrations at the receptor site, as well as in other tissues. As the con-centration of drug in plasma increases, the concentration of drug in most tissues will increase proportionally. Similarly, if the plasma concentration of a drug is decreasing, the concentration in tissues will also decrease. Is a simplified plot of the drug con-centration versus time profile after an intravenous drug dose and illustrates this concept

The property of kinetic homogeneity is important for the assumptions made in clinical pharmacokinetics. It is the foundation on which all therapeutic and toxic plasma drug concentrations are established. That is, when studying concentrations of a drug in plasma, we assume that these plasma concentrations directly relate to concentrations in tissues where the disease process is to be modified by the drug (e.g., the central nervous system in Parkinson’s disease or bone in osteomyelitis). This assumption, however, may not be true for all drugs.