Fraction Absorbed And Presystemic Availability Components

From an Oral Dose to the Systemic Circulation

Absolute oral bioavailability summarizes how much of an administered dose reaches the systemic circulation as unchanged drug. That total is useful, but it can combine several distinct losses: some drug may not be absorbed, some absorbed drug may not pass the intestinal wall unchanged, and some drug reaching the liver may be removed before entering the systemic circulation. Separating these steps helps explain how different mechanisms can produce the same total bioavailability.

The decomposition is a sequential accounting identity. It describes the fraction remaining after each step, not three alternative estimates of the same quantity. The component values are conditional fractions: each applies to the drug that reaches its corresponding step.

Under this definition, loss before absorption affects \(F_a\); loss in the intestinal wall before portal delivery affects \(F_g\); and hepatic first-pass loss affects \(F_h\). For example, if 80% of a dose is absorbed, then \(F_a=0.80\). If 75% of that absorbed amount reaches portal blood unchanged, then \(F_g=0.75\), not 0.75 of the original dose.

Each component is between zero and one. A value of one means no loss at that particular step; a value below one means that some drug is lost before the next step. For an intravenous reference route, these oral presystemic steps are bypassed, so they do not form a corresponding oral decomposition.

Why the Components Multiply

Consider an oral dose \(D\). The amount absorbed is \(D F_a\). Of that amount, the quantity reaching portal blood unchanged is \(D F_a F_g\). Of the portal amount, the quantity reaching the systemic circulation unchanged is \(D F_a F_g F_h\). Dividing the final amount by the original dose gives total bioavailability.

This is a product of sequential fractions, not a sum. Adding the components would count fractions measured at different stages as if they were separate contributions to systemic delivery. The multiplication also makes clear why a small value at any one step can substantially limit the overall result.

Worked Examples

What a Total Bioavailability Estimate Can and Cannot Say

An absolute bioavailability estimate obtained from oral and intravenous exposure is an estimate of the overall \(F\), subject to the assumptions of that comparison. It does not by itself measure \(F_a\), \(F_g\), or \(F_h\) separately. The equation \(F=F_aF_gF_h\) gives one relation among three unknown quantities. Many triples of component values can yield the same product, as the worked example shows.

The decomposition is most directly understood as a mass-balance model with consistent definitions and a common unchanged-drug endpoint. Each fraction must refer to the amount reaching its own stage, and the stages must be ordered consistently. If component values are proposed from different settings, drugs, or definitions, multiplying them may not describe any single dosing condition.

A useful justification for the product is to define the amount entering each stage. Let \(A_0=D\) be the oral dose, \(A_1=D F_a\) the amount absorbed, \(A_2=A_1F_g\) the amount delivered to portal blood unchanged, and \(A_3=A_2F_h\) the amount delivered systemically unchanged. Substitution gives the complete chain:

Since \(F=A_3/D\), division by the administered dose yields \(F=F_aF_gF_h\). The derivation depends on the amounts and fractions referring to the same dose and on each fraction being conditional on entry to its stated stage.

Diagnostics, Alternatives, and Limitations

Before interpreting a component decomposition, check that each value is a fraction on the zero-to-one scale and that the product agrees with the stated total. If values are reported as percentages, convert them to fractions before multiplying. For instance, 80%, 75%, and 60% correspond to 0.80, 0.75, and 0.60; their product is 0.36, not 36, and the total is 36%.

A mismatch between the product and a reported total may indicate arithmetic error, inconsistent units or definitions, rounding, or estimates that do not refer to the same conditions. A component above one cannot be interpreted as a fraction under this definition without revisiting how it was estimated or labelled. Likewise, do not treat the product as evidence that an observed change in total bioavailability arose at a particular stage.

The main alternative is to report total bioavailability alone. That is often the appropriate summary when the question concerns overall systemic delivery and component information is unavailable. A component decomposition can support a mechanistic explanation, but only when the component values have a defensible basis. It adds detail; it does not make the total estimate more precise or identify the cause of a change automatically.

The simple product is a stage-by-stage accounting description. In practice, component values may be difficult to measure separately, and estimates may be uncertain. If component values vary across individuals, the average of individual products need not equal the product of the average component values. In general, \(E(F_aF_gF_h)\) is not necessarily \(E(F_a)E(F_g)E(F_h)\), particularly when the components vary together. Therefore, a product formed from population-average components should not automatically be interpreted as average total bioavailability.

A common mistake is to conclude that if total bioavailability is 36%, then exactly 64% of the dose was “not absorbed.” That confuses absorption with overall systemic delivery. In the first worked example, 80% was absorbed, but only 36% reached the systemic circulation unchanged because additional losses occurred after absorption. The correction is to distinguish \(F_a\), the absorbed fraction, from \(F\), the product of all three sequential fractions.