The apparent volume of distribution is best described as which of the following?

• A. A precise anatomical space
• B. A measure of distribution that can be very high for lipophilic drugs and is not a true physiological space
• C. A fixed volume equal to plasma volume
• D. A constant equal to extracellular fluid

Answer: (B)

The apparent volume of distribution is a calculated parameter that links the amount of drug in the body to the plasma concentration; it is not a real anatomical space. For drugs that are highly lipophilic and distribute into tissues, this value can become very large because the drug resides outside the plasma in many tissues, making the ratio appear as if it were a volume much larger than any physical compartment. Thus it isn’t a true physiological space, just an “apparent” volume that helps describe how extensively a drug distributes relative to the plasma. It is not a fixed value tied to plasma volume, nor a constant equal to extracellular fluid, and it isn’t a precise anatomical space.