Apparent Volume of Distribution
What is the Apparent Volume of Distribution?
When we administer a drug in the body, it spreads to various body areas. Different drugs spread to different extend. As it spreads more and more, its plasma concentration falls. So we can know how far the drug is spreading, based on how much it is getting diluted in the plasma. The volume of that space is the apparent volume of distribution (aVd). We can use the following formula for this:
Why Is It Called "Apparent" Volume?
Because we are making an assumption based only on plasma concentration. We are not measuring the actual spread in all body compartments.
It gives a rough idea about how deep the drug is spreading in the body.
< 5 L suggests: the drug is mostly staying in the blood. e.g. heparin.
About 15 L suggests: the drug is spreading into the interstitium, but not spreading into the cells. e.g. gentamicin.
Up to 42 L suggests: the drug is entering the cells also. e.g. phenytoin.
> 42 L suggests: the drug is getting sequestered in deeper tissues. e.g. chloroquine
It's a volume that is apparently there to contain all the drug in the body at the same concentration as in the plasma.