Which statement about the volume of distribution (Vd) is true?

Which statement about the volume of distribution (Vd) is true?

• A. Vd represents the apparent distribution space and can be larger than total body water for lipophilic drugs.
• B. Vd is a fixed property and does not change with dose.
• C. Vd is determined solely by plasma volume.
• D. Vd cannot be greater than any bodily fluid volume.

Answer: (A)

Volume of distribution is the apparent space in which the total amount of drug would need to be distributed to produce the observed plasma concentration. It reflects how extensively a drug spreads from the blood into tissues. For lipophilic drugs, this distribution into fat and other tissues can be substantial, so the amount present in the body can be large even when plasma levels are low. That makes the Vd an apparent volume that can exceed the physical volumes of body water. In other words, the concept captured here is that Vd is not a fixed, physical container but a measure of distribution; lipophilic drugs often have very large Vd values because they partition into tissues beyond the plasma. The other statements don’t fit because Vd is not fixed and can vary with dose due to nonlinear distribution and binding, it is not determined solely by plasma volume since tissue distribution and binding govern much of it, and Vd is not limited by any bodily fluid volume since it is an apparent volume describing distribution into all compartments.

Volume of distribution is the apparent space in which the total amount of drug would need to be distributed to produce the observed plasma concentration. It reflects how extensively a drug spreads from the blood into tissues. For lipophilic drugs, this distribution into fat and other tissues can be substantial, so the amount present in the body can be large even when plasma levels are low. That makes the Vd an apparent volume that can exceed the physical volumes of body water. In other words, the concept captured here is that Vd is not a fixed, physical container but a measure of distribution; lipophilic drugs often have very large Vd values because they partition into tissues beyond the plasma.

The other statements don’t fit because Vd is not fixed and can vary with dose due to nonlinear distribution and binding, it is not determined solely by plasma volume since tissue distribution and binding govern much of it, and Vd is not limited by any bodily fluid volume since it is an apparent volume describing distribution into all compartments.