What is the difference between the volume of distribution at steady state (Vdss) and Vd?

• A. Vd is the apparent volume at equilibrium across compartments; Vdss is a time-specific estimate.
• B. Vdss is the apparent volume when distribution is at equilibrium; Vd is often an initial or time-specific estimate before full distribution.
• C. Vdss and Vd are always equal.
• D. Vdss represents body water, while Vd is plasma volume.

Answer: (B)

The key idea is that the volume of distribution changes with how far the drug has distributed in the body. Vd is the apparent volume you get at a particular time, often during the distribution phase after an IV dose—the drug hasn’t yet equilibrated between plasma and tissues, so this “volume” can be smaller and time-specific. Vdss, by contrast, is the apparent volume once the drug has distributed to equilibrium between compartments (steady-state). At that point, the amount in the body and the plasma concentration reflect true, full distribution, and Vdss is the stable value used for dosing calculations in linear pharmacokinetics.

So the correct statement is that Vdss is the apparent volume when distribution is at equilibrium, while Vd is often an initial or time-specific estimate before full distribution. The other options misstate which parameter reflects equilibrium, or wrongly tie Vdss to body water or imply they’re always equal.