This is the total amount of the drug excreted unchanged in urine when the administered drug dose is completely eliminated. The key word here is "unchanged". This cumulative drug amount does not include the drug which is metabolized and then the metabolite is excreted in urine.

The amount of the drug excreted unchanged in urine is determined by the renal clearance and the total body clearance of the drug. (Fraction of dose secreted in urine = CL_R/CL_T)

The total amount of the drug excreted unchanged in urine after an iv drug administration is determined by adding the amount of the drug excreted in each urine collection until all the drug is completely eliminated.

The fraction of bioavailable dose excreted in urine is determined from the ratio of the renal clearance to the CL_T

By rearrangement:

and because

then

Bioavallability is a measure of the fraction of the administered dose that reaches the systemic circulation.

Absolute bioavailability is the fraction of the total dose that reaches the systemic circulation. It can have values between zero when the drug is not absorbed at all to 1 when all the administered dose reaches the systemic circulation. The absolute bioavailability can be expressed in terms of percentage (0% - 100%).

The relative bioavailability is the bioavailability of a drug product relative to the bioavailability of another drug product (reference standard preparation). The relative bioavailability can have any value above zero. It can be more than one when the product under investigation has bioavailability higher than that of the reference standard.

It is the integral of the plasma concentration-time profile from time zero to time infinity.

It has units of mass-time/volume.

The area under the curve after an iv administration is dependent on the administered dose and the total body clearance.