In pharmacology, the volume of distribution (👁 {\displaystyle V_{D}}
, also known as apparent volume of distribution or volume of dilution^[1]) is the theoretical volume that would be necessary to contain the total amount of an administered drug at the same concentration that it is observed in the blood plasma.^[2]

Roughly speaking, the 👁 {\displaystyle V_{D}}
, as a property of a drug, measures the degree to which it is distributed in body tissue rather than the blood plasma. Drug properties which cause high 👁 {\displaystyle V_{D}}
include high lipid solubility (non-polarity), low rates of ionization, or low plasma protein binding capabilities. Disease states which increase 👁 {\displaystyle V_{D}}
include kidney failure (due to fluid retention) and liver failure (due to altered body fluid and plasma protein binding). Conversely, dehydration may decrease 👁 {\displaystyle V_{D}}
.

The initial volume of distribution describes blood concentrations prior to attaining the apparent volume of distribution and uses the same formula.

Suppose one administers an amount of drug 👁 {\displaystyle D}
intravascularly, then measures the drug concentration in blood 👁 {\displaystyle C_{0}}
(assuming enough time has elapsed for the drug to distribute, but not enough time for elimination). The volume of distribution is the quotient:

👁 {\displaystyle V_{D}={\frac {D}{C_{0}}}}

If the drug remains entirely intravascularly, 👁 {\displaystyle V_{D}}
will be identical to the blood volume 👁 {\displaystyle V_{blood}}
. However, if a drug diffuses out of the intravascular space into the tissues or interstitium, the measured concentration will be lower-than-expected compared to a hypothetical intravascular-only drug. Therefore, 👁 {\displaystyle V_{D}>V_{blood}}
, with a higher 👁 {\displaystyle V_{D}}
value corresponding to a greater tendency for the drug to exit the intravascular space.

One clinical utility is that the dose required 👁 {\displaystyle D}
to achieve a target plasma concentration 👁 {\displaystyle C_{0}}
can be determined if the 👁 {\displaystyle V_{D}}
for that drug is known.

The 👁 {\displaystyle V_{D}}
is not a physiological value; it is more a reflection of how a drug will distribute throughout the body depending on several physicochemical properties such as solubility, charge, size, etc.

The unit for 👁 {\displaystyle V_{D}}
may be reported extensively in litres (for a patient of given weight), or intensively as litres-per-kilogram.

The 👁 {\displaystyle V_{D}}
may also be used to determine how readily a drug will displace into the body tissue compartments relative to the blood:

👁 {\displaystyle {V_{D}}={V_{P}}+{V_{T}}\left({\frac {f_{u_{P}}}{f_{u_{T}}}}\right)}

Where:

• 👁 {\displaystyle V_{P}}
  : plasma volume
• 👁 {\displaystyle V_{T}}
  : apparent tissue volume
• 👁 {\displaystyle f_{u_{P}}}
  : fraction unbound in plasma
• 👁 {\displaystyle f_{u_{T}}}
  : fraction unbound in tissue

For example, chloroquine has much greater affinity for body fat than blood, resulting in a 👁 {\displaystyle V_{D}\approx 250L/kg}
^[3] compared to 👁 {\displaystyle V_{blood}\approx 0.08L/kg}
.^[4]

• Tutorial on volume of distribution
• Overview at icp.org.nz Archived 2008-10-14 at the Wayback Machine
• Overview at cornell.edu
• Overview at stanford.edu
• Overview at boomer.org