For an intravenous bolus administration, which statement about loading dose is true?

• A. Loading dose for an IV bolus depends on volume of distribution and is independent of bioavailability.
• B. Loading dose for an IV bolus depends on bioavailability but not on volume.
• C. Loading dose for an IV bolus depends on absorption rate.
• D. Loading dose for an IV bolus depends only on clearance.

Answer: (A)

For an IV bolus, the goal of a loading dose is to achieve a desired plasma concentration immediately by delivering enough drug to occupy the body's volume of distribution. The amount needed is D = Vd × Ctarget / F. Since an IV bolus delivers drug directly into the bloodstream, bioavailability F equals 1, so the equation becomes D = Vd × Ctarget. In other words, the loading dose depends on how extensively the drug distributes (volume of distribution) and on the target concentration, not on absorption or bioavailability (which is already complete) and not on the rate of absorption (there is none for an IV bolus). Clearance doesn’t set the initial amount to reach the target concentration; it influences how the concentration declines over time after the bolus and therefore affects maintenance dosing, not the loading dose itself.

That’s why the statement that the loading dose depends on volume of distribution and is independent of bioavailability is the best one.