What is bioavailability (F) and how is it calculated for an extravascular dose?

• A. F is the fraction of the administered dose that reaches systemic circulation.
• B. F = (AUCpo × DoseIV) / (AUCIV × DosePo).
• C. F is the ratio of AUCIV to AUCpo.
• D. F equals DoseIV divided by DosePo.

Answer: (B)

Bioavailability, F, is the fraction of the administered dose that reaches systemic circulation. For an extravascular dose, not all of the drug makes it into the blood due to incomplete absorption and first-pass metabolism, so we compare the systemic exposure from the extravascular route to that from an intravenous dose, which is considered fully available. Exposure is captured by the area under the concentration-time curve (AUC). Since the two routes may involve different doses, we normalize by dose to make a fair comparison. The absolute bioavailability is calculated as F = (AUC_extravascular × Dose_IV) / (AUC_IV × Dose_extravascular). This formula shows how much of the extravascular dose actually becomes available in the systemic circulation. The other statements don’t provide this dose-normalized exposure calculation, so they don’t correctly quantify F.