True or false: It is impossible to determine the total amount of drug present in the body by direct methods.

True or false: It is impossible to determine the total amount of drug present in the body by direct methods.

• A. True
• B. False
• C. Only with biopsy
• D. Only with imaging

Answer: (A)

The amount of drug in the body cannot be determined directly because the body consists of many compartments—plasma, interstitial fluid, intracellular spaces, and various tissues—each with different levels of drug distribution and binding. To know the total amount, you’d have to measure drug concentrations in every compartment, which is not feasible in practice due to invasiveness, variability, and ethical concerns. What we can do instead is estimate the total amount using pharmacokinetic models and plasma data. By analyzing how plasma concentration changes over time, we derive parameters like the apparent volume of distribution (Vd) and clearance, and the total amount is inferred from these models (A = C × Vd, conceptually), not measured directly. Biopsy would sample only a small part of the body, and imaging provides distribution patterns rather than an exact total mass, so neither yields a direct total-body quantity.

The amount of drug in the body cannot be determined directly because the body consists of many compartments—plasma, interstitial fluid, intracellular spaces, and various tissues—each with different levels of drug distribution and binding. To know the total amount, you’d have to measure drug concentrations in every compartment, which is not feasible in practice due to invasiveness, variability, and ethical concerns. What we can do instead is estimate the total amount using pharmacokinetic models and plasma data. By analyzing how plasma concentration changes over time, we derive parameters like the apparent volume of distribution (Vd) and clearance, and the total amount is inferred from these models (A = C × Vd, conceptually), not measured directly. Biopsy would sample only a small part of the body, and imaging provides distribution patterns rather than an exact total mass, so neither yields a direct total-body quantity.