How is bioavailability mathematically calculated?

Bioavailability is an important concept in pharmacokinetics that describes the proportion of a drug that enters the systemic circulation when introduced into the body, compared to when it is administered intravenously (IV). Mathematically, bioavailability is calculated using the area under the curve (AUC) from the plasma concentration-time profile.

The correct calculation for bioavailability is determined by the ratio of the AUC after oral administration to the AUC after IV administration. This reflects how much of the drug reaches the systemic circulation through the oral route compared to the direct entry through IV. Since IV administration bypasses first-pass metabolism and absorption issues, it serves as a baseline for comparison.

In terms of the mathematical expression, the AUC following oral administration represents the total drug exposure from that route, while the AUC following IV administration indicates the total exposure without any losses. Thus, the bioavailability is expressed as AUC oral divided by AUC IV, providing a clear understanding of how much of the orally administered drug is available in comparison to the IV route.

The other options do not accurately reflect how bioavailability is calculated. The second choice, for instance, suggests that AUC IV is divided by AUC oral, which would not provide a meaningful ratio for bio