NAPLEX (North American Pharmacist Licensure Examination) Practice Exam

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How is volume of distribution (Vd) calculated?

amount of drug in plasma/ concentration of drug in body
amount of drug in body/ concentration of drug in plasma
concentration of drug in plasma / amount of drug in body
concentration of drug in body / amount of drug in plasma

The correct answer is: amount of drug in body/ concentration of drug in plasma

Volume of distribution (Vd) represents the extent to which a drug distributes itself throughout the body. It is calculated by taking the amount of drug in the body and dividing it by the concentration of drug in the plasma. The correct formula for Vd is Vd = amount of drug in body / concentration of drug in plasma. This calculation is not equivalent to the other options provided. Option A flips the numerator and denominator, which would result in a different value. Option C is similar to option A but with the units flipped, which would again produce a different Vd value. Option D uses the concentration of the drug in the body, which is not the same as the amount of drug in the body used in the correct formula. Overall, Vd helps to determine how a drug is distributed throughout the body and is calculated using the correct formula of Vd = amount of drug in the body / concentration of drug in the plasma.