Volume of Distribution (Vd)
Definition
The apparent volume in which a drug is distributed in the body, calculated as dose divided by plasma concentration at time zero. Low Vd (<0.5 L/kg) means the drug stays largely in plasma; high Vd (>3 L/kg) means extensive tissue binding. Vd governs how a drug behaves in post-mortem redistribution.
- Low Vd threshold
- Below 0.5 L/kg means the drug stays largely in plasma
- High Vd threshold
- Above 3 L/kg indicates extensive tissue binding
- Clinical significance
- Vd governs how a drug behaves in post-mortem redistribution
Common questions
What does a high volume of distribution tell you about a drug?+
A high Vd (above 3 L/kg) means the drug is binding extensively to tissues beyond the bloodstream. This indicates a lipophilic (fat-soluble) drug that concentrates in tissues rather than staying in plasma. It's particularly pronounced in obese individuals.
How is volume of distribution calculated?+
Vd is calculated as the dose divided by the plasma concentration at time zero. This gives the apparent volume into which a drug has distributed throughout the body. It's a theoretical volume, not a measurable anatomical space.
Why does body composition matter for volume of distribution?+
Lipophilic drugs preferentially distribute into fatty tissues. In obese subjects, the same drug dose reaches a higher Vd because it has more fat to bind to. Lean subjects show lower Vd for the same drug, resulting in higher plasma concentrations.
Related terms
- 6-Monoacetylmorphine (6-MAM)
- A transient diagnostic metabolite of heroin (diacetylmorphine) formed by the rapid hydrolysis of one acetyl group. Half-life is 6 to 25 minutes...
- Benzoylecgonine (BE)
- The major hydrolysis metabolite of cocaine, formed by carboxylesterase action on the methyl ester of cocaine. Half-life is around 6 hours in...
- First-Pass Metabolism
- Hepatic and gut wall biotransformation of orally absorbed drug before it reaches systemic circulation. Bypassed by IV, intramuscular, inhalation and most dermal...
- Idiosyncrasy
- A qualitatively abnormal response to a normal dose, traceable to genetic polymorphism in metabolising enzymes, transporters or receptor targets. Not the same...
- LD50
- Median lethal dose; the dose of a substance that kills 50% of a test population, expressed as mg/kg body weight. The classical...
- Pharmacogenomics
- Study of how inherited variation in drug-metabolising genes (CYP2D6, CYP2C19, NAT2, G6PD) changes drug response. The Indian Genome Variation Consortium and ICMR...
- Phase I Metabolism
- Functionalisation reactions (oxidation, reduction, hydrolysis) that introduce or expose a polar group on the drug molecule, mostly catalysed by cytochrome P450 isoenzymes...
- Phase II Metabolism
- Conjugation reactions (glucuronidation, sulphation, glutathione conjugation, acetylation, methylation, glycine conjugation) that attach a polar endogenous group to the drug or its Phase...
- Post-Mortem Redistribution (PMR)
- The phenomenon where drug and poison concentrations shift between body compartments after death, generally moving out of solid organs back into the...
- Therapeutic, Toxic and Lethal Ranges
- Plasma concentration ranges that guide interpretation, derived from population studies and case reports. They are guidelines, not absolutes. Tolerance, idiosyncratic response, polypharmacy...
- Tolerance
- Reduced response to a given dose after repeated exposure. Pharmacokinetic tolerance is enzyme induction or transporter upregulation; pharmacodynamic tolerance is receptor downregulation...
Explained in these topics
- Drug Metabolism, Distribution and Metabolite IdentificationThe apparent volume in which a drug is distributed in the body, calculated as dose divided by plasma concentration at time zero. Low Vd (<0.5 L/kg) means the d...
- Factors Affecting the Intensity of PoisoningApparent volume into which a drug distributes after absorption. Lipophilic drugs have a high Vd in obese subjects and a low Vd in lean subjects, shifting plasm...