A drug has a volume of distribution of 700 L in a 70 kg patient. This implies that the drug is:

• A Confined to plasma (plasma volume ~3 L) and is highly protein-bound
• B Limited to the extracellular fluid space (~14 L)
• C Distributed evenly across total body water (~42 L) with minimal tissue binding
• D Extensively distributed to peripheral tissues, with the majority outside the vascular compartment ✓

Explanation

Volume of distribution (Vd) is a mathematical constant representing the apparent volume needed to contain the total amount of drug at the observed plasma concentration. A Vd of 700 L far exceeds the total body volume (70 L for a 70 kg person), indicating that the drug extensively partitions into peripheral tissues (lipid, protein binding in tissues). The low plasma concentration relative to the tissue-bound amount creates this apparent 'excess' volume. Chloroquine, digoxin, and amiodarone have very high Vd values. Low Vd (3–5 L) indicates plasma confinement.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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