Before diving into the difference between absolute bioavailability and relative bioavailability, let us learn some basics of the bioavailability concept. Bioavailability is a critical pharmacokinetic parameter that influences the drug’s therapeutic efficacy and safety. It is the rate and extent at which absorption of active moiety from a drug product takes place and becomes available at the site of action.

Regulatory agencies like the FDA require the submission of bioavailability data in their applications to market their new drug products. Bioavailability is mostly determined using the Area Under the Plasma concentration vs time curve. Let us see the difference between absolute bioavailability and relative bioavailability.

Absolute Bioavailability: It is a measure of the bioavailability of a drug when administered non-intravenously and compared with the same drug administered intravenously. As we all know intravenous route of administration drug reaches systemic circulation directly and it has 100 percent bioavailability.

The non-intravenous or extravascular routes include oral, rectal, subcutaneous, inhalational, etc.

The formula for absolute bioavailability is

Fabs =  (AUCnon−iv​ / AUCiv) x (Dose iv / Dosenon−iv​) 

Where

• $AUC$ is the Area Under the plasma Concentration-time Curve.
• iv refers to intravenous administration.
• non−iv refers to any other route of administration.

​ Relative Bioavailability is the comparing the bioavailability of the same drug from different formulations or different routes of administration other than intravenous. This measure is used to compare a test formulation to a reference formulation. Here relative bioavailability compares different non intravenous routes like oral, rectal, inhalational, subcutaneous etc.

The formula for calculating the relative bioavailability is

F-relative  = AUC test / AUC reference x Dose of reference / Dose of test

Here

• $AUC$ is the Area Under the plasma Concentration-time Curve.
• test refers to the test formulation or route.
• reference refers to the reference formulation or route.

​