Fraction Absorbed Estimate

What is this formula?

The fraction absorbed estimate quantifies the proportion of an administered dose that is absorbed across biological membranes into the body.

It is one of the fundamental parameters used to characterize drug absorption and is often represented by Fa in pharmacokinetic models.

A value of 1 indicates complete absorption, while lower values indicate incomplete absorption.

When to use it

Use this formula when the absorbed amount of drug and the administered dose are known or can be estimated.

It is commonly used in absorption studies, oral drug development, physiologically based pharmacokinetic models, and bioavailability research.

Example

A patient receives an oral dose of 500 mg. Analysis indicates that 350 mg was absorbed.

Data:

Aabs = 350 mg

Dose = 500 mg

Formula:

Fa = Aabs/Dose

Substitution:

Fa = 350/500

Result:

Fa = 0.70

Fraction absorbed = 70%

Applications

- Drug absorption studies

- Oral formulation development

- Pharmacokinetic modeling

- Bioavailability assessment

- Physiologically based pharmacokinetic models

Note

The fraction absorbed is often estimated rather than measured directly. Experimental methods may rely on mass balance studies, urinary recovery, plasma concentration profiles, or pharmacokinetic modeling. Fa represents absorption into the body and should not be confused with bioavailability, which may also be affected by first-pass metabolism and presystemic elimination.