ABSTRACT
In the expression, dose represents the amount of drug that is present in the body. Cp represents the plasma concentration obtained immediately after the dose. For example, if a 100-mg dose of gentamicin produces a plasma concentration of 5 mg/L, the apparent volume of distribution is 20 L: 20 L = 100 mg 5 mg/L. The apparent volume of 20 L does not represent an actual anatomical volume, but the apparent fluid volume that is necessary to contain the entire drug dose at the same concentration as is found in the plasma. The apparent Vd is used to characterize the extent of drug distribution throughout the body. A drug with a small Vd may remain primarily confined to the vascular space. In contrast, a drug with a large Vd may leave the vascular space and undergo extensive tissue distribution. In clinical practice the apparent Vd is used to calculate the magnitude of drug doses.
