Which formula gives absolute bioavailability F in a clinical study?

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

Answer: (A)

Absolute bioavailability tells you what fraction of the oral dose actually reaches systemic circulation compared with a directly injected IV dose. The proper way to quantify this uses both the plasma exposure (AUC) and the actual doses given by each route: F = (AUC_po × Dose_iv) / (AUC_iv × Dose_po).

The AUCs capture the extent of systemic exposure after each route, so multiplying the oral AUC by the IV dose and dividing by the IV AUC and the oral dose normalizes for differences in dosing between routes. This yields a dimensionless fraction between 0 and 1, representing the portion of the oral dose that becomes systemically available.

The other options don’t fit: one would invert the ratio and misuse the dose terms; another would ignore dose differences entirely; and the last uses only dose information without any exposure data.