Variables
Description
What is this formula?
Absolute bioavailability measures the fraction of an administered drug dose that reaches the systemic circulation unchanged compared with intravenous administration.
Because intravenous administration delivers 100% of the dose directly into circulation, it serves as the reference standard for determining bioavailability.
A value of F = 1 indicates complete systemic availability, while lower values indicate incomplete absorption and/or first-pass metabolism.
When to use it
Use this formula when both intravenous and extravascular pharmacokinetic studies are available and their AUC values have been measured.
It is commonly used during drug development, formulation comparison, regulatory studies, and pharmacokinetic research.
Example
A drug is administered both intravenously and orally.
Data:
AUCpo = 80 mg·h/L
AUCiv = 100 mg·h/L
Dosepo = 200 mg
Doseiv = 100 mg
Formula:
F = (AUCpo × Doseiv)/(AUCiv × Dosepo)
Substitution:
F = (80 × 100)/(100 × 200)
F = 8000/20000
Result:
F = 0.40
Absolute bioavailability = 40%
Applications
- Drug development
- Oral formulation evaluation
- Regulatory pharmacokinetic studies
- Assessment of first-pass effects
- Comparison of administration routes
Note
Absolute bioavailability is an observational pharmacokinetic parameter that depends on absorption, gastrointestinal stability, presystemic metabolism, and formulation characteristics. Different study designs and sampling schedules may produce slightly different estimates. Intravenous administration is assumed to provide complete systemic availability.