Rational therapeutic drug monitoring.

The simple act of ordering a serum drug level does not guarantee that the information will be meaningful or useful. The interpretation of serum concentrations can be profoundly influenced by such factors as the timing of the sample, the patient's clinical state, the drug's pharmacokinetics and metabolism, and the tube type and analytic methodology used. The likelihood of obtaining clinically meaningful and useful results can be maximized when these factors are taken into account.