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 fraction remaining of the normal kidney function. This is estimated by comparing the kidney function of the patient with the normal kidney function.

The creatinine clearance is a diagnostic test which is highly correlated with the kidney function. So, the creatinine clearance is used as a measure of the kidney function.

The kidney function can range from 1 when the patient's kidney function is normal to 0 when the patient's kidney function is completely impaired (anureic patients).

The creatinine clearance can be determined directly from urinary excretion rate and serum creatinine. Also, the creatinine clearance can be estimated from serum creatinine, age, and weight.