Clinical pharmacokinetics relies on a relationship between the pharmacological effects of a drug and a measurable concentration of the drug (e.g., in blood or plasma). For some drugs, no clear or simple relationship has been found between pharmacological effect and concentration in plasma, whereas for other drugs, routine measurement of drug concentration is impractical as part of therapeutic monitoring. In most cases, the concentration of drug at its sites of action will be related to the concentration of drug in the systemic circulation. The pharmacological effect that results may be the clinical effect desired, or an adverse or toxic effect. Clinical pharmacokinetics provides a framework within which drug dose adjustments can be made.
The physiological and pathophysiological variables that dictate adjustment of dosage in individual patients often do so as a result of modification of pharmacokinetic parameters. The four most important parameters governing drug disposition are clearance, a measure of the body's efficiency in eliminating drug; volume of distribution, a measure of the apparent space in the body available to contain the drug; elimination tm, a measure of the rate of removal of drug from the body; and bioavailability, the fraction of drug absorbed as such into the systemic circulation.
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