It is the fraction of the administered iv dose which is excreted unchanged in urine. The amount of the drug excreted in urine is the total amount of the unchanged drug found in the urine when urine is collected from the time of drug administration until all the drug is eliminated from the body. If the drug is metabolized and then the metabolite is eliminated in urine, this is not included in the amount of drug excreted in urine.

The fraction ( f) is the ratio of the total amount excreted in urine to the iv administered dose. This fraction can range from 1 when all the drug is excreted in urine to 0 when the drug is not excreted in urine.

This fraction is determined from the ratio of:

- The renal elimination rate constant to the overall elimination rate constant

- The renal clearance to the total body clearance

- The elimination rate of the drugs that are excreted by the kidney to larger extent ( larger f) are affected more by kidney dysfunction. These drugs have slower rate of elimination and longer elimination half lives.

- Drugs that are excreted in urine to larger extent ( larger f) require larger reduction in their doses in patients with renal dysfunction to achieve therapeutic drug concentrations.