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 first-order renal excretion rate constant is the rate constant for the renal excretion of the drug. Larger renal excretion rate constant leads to higher rate of renal drug excretion.

This rate constant represents drug excretion by the kidney. The renal excretion rate constant cannot be bigger than the overall elimination rate constant (k). When the drug is excreted entirely unchanged in urine by the kidney, the renal clearance will be equal the total body clearance and the renal excretion rate constant will be equal to the overall elimination rate constant.

The first-order renal excretion rate constant has units of time^-1.

The first-order elimination rate constant is the rate constant for the elimination of the drug from the body. The elimination rate constant and the half life are dependent on the total body clearance and the volume of distribution of the drug.

This rate constant represents drug elimination through all routes of drug elimination such as metabolism, renal excretion, and other routes.

The first-order elimination rate constant has units of time^-1.

It is the volume of the plasma or the blood that is completely cleared from the drug per unit time by the kidney. It has units of volume/time.

The renal clearance cannot exceed the CL_T. However it can be equal to the CL_T when the drug is excreted completely unchanged by the kidney.

The renal clearance can be determined from the renal excretion rate over a short interval and the average plasma concentration during this interval.

The total body clearance is the volume of the plasma or blood which is completely cleared from the drug per unit time. It has units of volume/time.

The CL_T for a drug is constant within a patient (dose and concentration independent) when the elimination processes follow first-order kinetics.

The total body clearance is a measure of the efficiency of all eliminating organs in eliminating the drug and it is the sum of all organ clearances (i.e. CL_T is the sum of the renal clearance, hepatic clearance and all other organ clearances).

The elimination rate constant and the half life (the dependent pharmacokinetic parameters) are dependent on (is determined from) the total body clearance and the volume of distribution (the independent pharmacokinetic parameters).