Absolute Bioavailability

Absolute bioavailability is the fraction of an administered dose that reaches the systemic circulation unchanged when compared against intravenous administration, which by definition delivers the entire dose to the blood. It is expressed as the bioavailable fraction F, ranging from zero to one, and it quantifies the cumulative loss a drug suffers between the dosing site and the bloodstream.

Definition

Absolute bioavailability is the dose-corrected ratio of systemic exposure (area under the plasma concentration-time curve) after an extravascular dose to the exposure after an intravenous dose of the same drug, yielding the fraction F of the dose that reaches the systemic circulation intact.

Scope

The entry covers the definition of absolute bioavailability, the intravenous reference that anchors it, the dose-normalised area-under-the-curve calculation used to estimate it, and the absorption and first-pass losses that determine its value. It treats absolute bioavailability as a measurement concept in biopharmaceutics rather than as dosing guidance.

Core questions

• What fraction of an extravascular dose actually reaches the systemic circulation?
• Why is intravenous administration the reference for absolute bioavailability?
• How is F estimated from AUC values and the respective doses?
• Which losses reduce F below one?

Key concepts

• Bioavailable fraction F
• Intravenous reference (F = 1)
• Dose-normalised AUC ratio
• Fraction absorbed
• First-pass loss as a determinant of F
• Interpatient variability in F

Mechanisms

Because an intravenous dose enters the bloodstream completely, comparing systemic exposure after an extravascular route to that after intravenous dosing isolates everything lost along the way. Absolute bioavailability is computed as the ratio of dose-normalised areas under the concentration-time curves for the two routes. The resulting fraction reflects the product of the fraction that escapes the dosage form and dissolves, the fraction that permeates the gut wall, and the fraction that survives presystemic metabolism in gut and liver; incomplete absorption or extensive first-pass extraction drives F well below one. Because absorption is often the rate-limiting and most variable step, drugs with low absorption tend to show greater interpatient variability in their bioavailability.

Clinical relevance

Absolute bioavailability explains why an intravenous and an oral form of the same drug can produce very different systemic exposures, and why some drugs are impractical to give by mouth. It is a reference quantity for understanding route-dependent exposure; it characterises a drug's behaviour and is not itself a basis for individual dosing decisions.

Evidence & guidelines

Absolute bioavailability is determined in dedicated pharmacokinetic studies that pair an extravascular dose with an intravenous reference, and large compilations of human intravenous pharmacokinetic data, such as the database analysed by Obach and colleagues, provide the reference parameters that make such comparisons possible. Hellriegel and colleagues documented that the extent of absorption is a major source of the between-subject variability seen in measured bioavailability.

History

Absolute bioavailability became measurable once plasma drug assays allowed the area under the concentration-time curve to be quantified for different routes in the same subjects. The intravenous-referenced fraction F entered standard pharmacokinetic teaching through textbooks such as Rowland and Tozer, and later large-scale collations of human intravenous data extended the reference base against which oral exposure is judged.

Key figures

• Malcolm Rowland
• Thomas Tozer
• R. Scott Obach

Seminal works

• obach-2008
• hellriegel-1996

Frequently asked questions

Why is intravenous dosing used as the reference for absolute bioavailability?

An intravenous dose bypasses absorption and first-pass loss and delivers the full dose to the systemic circulation, so it represents complete availability (F = 1) against which other routes are compared.

What does an absolute bioavailability of 0.3 mean?

It means that, after correcting for dose, only about 30 percent of the administered drug reached the systemic circulation intact, with the remaining 70 percent lost to incomplete absorption and presystemic elimination.