absolute bioavailability

Absolute bioavailability asks: compared with putting the drug straight into a vein, how much of an oral (or other non-IV) dose actually reaches the bloodstream? Intravenous injection skips absorption entirely — every milligram is delivered — so it is the gold-standard yardstick. Measuring against it tells you the true fraction your tablet or patch manages to deliver.

It is calculated by giving the same drug by the test route and by IV (usually in a crossover study) and comparing exposures, dose-corrected: F = (AUC_test / Dose_test) ÷ (AUC_IV / Dose_IV). The result is a fraction between 0 and 1 (or a percentage). Because the IV reference is by definition 100% available, the ratio captures all the losses along the oral route — incomplete dissolution and absorption plus first-pass metabolism.

A value well below 100% is common and not necessarily a flaw — many useful drugs have modest absolute bioavailability; it just has to be reproducible and accounted for in dosing. The honest caveat is that an IV formulation of the drug must exist and be safe to inject, which is not always feasible for poorly soluble or irritant molecules; in those cases only relative bioavailability can be measured.