Elimination Elimination is the process whereby the body rids itself of drug. Most drugs are eliminated unchanged by the kidney in the urine or are first metabolized by the liver and then excreted in the urine as drug metabolites. As stated above, renal clearance contributes to total body clearance of a drug and, for drugs like the aminoglycosides, represents a major portion of total body clearance. The rate of drug elimination is most easily followed by monitoring the decline in serum levels with time. Serum concentrations usually decline in an exponential fashion, such that a plot of the log of concentration vs. time would produce a straight line.
Elimination is also a major determinant of the time it takes to reach steady-state.
The presence of various disease states is an important source of variability between patients when monitoring therapeutic drugs with serum levels. Most pharmacokinetic parameters were originally described in normal volunteers, and so are not applicable to a diseased patient population. The conditions most studied for their impact on pharmacokinetics include hepatic, renal, and heart disease. Other conditions, such as pregnancy, can also affect drug disposition. Premature infants have larger volumes of distribution as well as immature mechanisms for renal and hepatic drug elimination.