How is oral bioavailability F calculated using AUC after PO and IV administration?

• A. F = (AUC_iv * Dose_po) / (AUC_po * Dose_iv)
• B. F = (AUC_po * Dose_iv) / (AUC_iv * Dose_po)
• C. F = AUC_po / AUC_iv
• D. F = Dose_po / Dose_iv

Answer: (B)

Oral bioavailability is the fraction of the oral dose that reaches systemic circulation. Since the AUC reflects overall exposure, you must compare exposure per unit dose for the two routes. The correct way is to normalize each AUC by its dose and then form the ratio: F = (AUC_po / Dose_po) ÷ (AUC_iv / Dose_iv). This simplifies to F = (AUC_po * Dose_iv) / (AUC_iv * Dose_po). This expression correctly accounts for differences in dose between the oral and IV administrations.

If you use AUCs alone without dosing normalization, you’d ignore how much was given, which misestimates the fraction that actually becomes systemic. The reciprocal form or any formula that omits the dose terms would lead you astray.