What is bioavailability (F) and how is it measured?

• A. F is the fraction of administered dose reaching systemic circulation; measured by comparing AUC after non-IV administration to IV administration.
• B. F is the rate of absorption; measured by time to peak concentration.
• C. F is the amount excreted unchanged; measured by urinary excretion.
• D. F is the fraction lost to first-pass metabolism; measured by hepatic clearance.

Answer: (A)

Bioavailability is the fraction of the administered dose that actually reaches the systemic circulation. It is assessed by comparing the body's exposure to the drug after a non-intravenous dose with the exposure after an intravenous dose, adjusting for any dose differences. In practice, F is calculated as the AUC after a non-intravenous route divided by the AUC after IV administration, multiplied by the ratio of the IV dose to the non-IV dose. Since IV dosing has F = 1, this comparison tells you how much of the non-IV dose makes it into circulation. This measure relies on the area under the plasma concentration–time curve (AUC). The other statements describe the rate of absorption (how quickly the drug enters the bloodstream), urinary excretion, or hepatic clearance—properties related to different PK concepts, not the fraction of the dose that becomes systemic.