volume of distribution (VD, VOD)

Introduction

An indication of the extent to which a drug is distributed outside of the blood compartment.

The value does not reflect an identifiable physiologic volume, but corresponds to the volume that would be required to contain all of the drug in the body at the same concentration as in the plasma.

Volume = dose(IV)/Co

where Co is the plasma concentration of the drug back- extrapolated to time zero

For a 70 kg human:

plasma volume = 3 L

blood volume = 5.5 L

extracellular water = 42 L

For drugs extensively bound to plasma proteins but not tissue components, volume of distribution approaches plasma volume.

For most drugs, the volume of distribution is far greater than any physiologic space.