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).

The formation clearance of the metabolite is the fraction of the drug total clearance which is responsible for the formation of the metabolite. It is the product of the drug CL_T and the fraction of the drug dose converted to the metabolite (f_m).

When all the administered drug dose is metabolized to one metabolite, the formation clearance should be equal to the CL_T of the drug (f_m = 1).

When multiple metabolites are formed by parallel metabolism, each metabolite will have its own formation clearance. The sum of the formation clearances for all metabolites should not be more than the drug total clearance.

However, the sum of the formation clearances of the metabolites will not be equal to the drug CL_T if the drug elimination involves pathways other than drug metabolism.

The half life is the time required for the amount of the drug or the plasma concentration of the drug to decrease by 50%. The half life has units of time.

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

The half life of a drug is constant within a patient (dose and concentration independent) when the drug elimination process follows first-order kinetics. However different patients may have different half lives for the same drug.

The half life is a very important parameter, because it indicates how fast the drug is eliminated from the body. Drugs with shorter half lives are eliminated faster than those with longer half lives.

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.

The first-order elimination rate constant of the metabolite is the rate constant responsible for the elimination of the metabolite from the body. This rate constant is dependent on the clearance and the volume of distribution of the metabolite. (CL_T(m)) / Vd_(m) = k_(m))

The elimination rate constant of the metabolite is the same when the metabolite is formed in vivo after the parent drug administration and after administration of the preformed metabolite.

This constant represents the metabolite elimination through all routes of metabolite elimination including secondary metabolism and renal excretion.

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