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.

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 reference standard should contain the same active ingredient as the test product under investigation.

The reference standard should be the formulation that has the highest bioavailability for the same active ingredient.

The reference standard should be administered by the same route as the formulation under investigation.

The reference standard for a drug is usually a formulation currently approved and marketed which is well accepted by the medical profession and has a long history of clinical use. The reference standard is usually the innovator's or the original manufacturer's product.