Bioavailability (F)
Bioavailability is the fraction of a dose that reaches the systemic circulation. With only oral data, F cannot be separated from clearance and volume; only CL/F and V/F can be estimated.
Definition
Bioavailability (F) is the fraction of an administered dose that reaches the systemic circulation unchanged. For an intravenous dose F = 1. It is estimated by comparing the AUC after an oral dose with the AUC after an intravenous dose, corrected for the dose.
Apparent parameters
If only oral data are available, F cannot be separated from the clearance and the volume. What can be estimated are the apparent clearance CL/F and the apparent volume V/F. This is a case of structural identifiability: F, CL and V are not identifiable separately, whatever the number of samples.
On this site
The warfarin PK/PD analysis uses an oral one-compartment model and states that bioavailability cannot be separated from clearance or volume with these oral data, so apparent CL/F and V/F are reported.
Frequently asked questions
Why does oral-only data give CL/F and not CL?
The concentration depends on dose × F, so a change in F is indistinguishable from the same change in CL and V; only the ratios CL/F and V/F are determined.
How is absolute bioavailability measured?
By comparing the AUC after an oral dose with the AUC after an intravenous dose in the same subjects, with a correction for the dose.