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).

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

The organ clearance cannot exceed the total body clearance. However it can be equal to the total body clearance when the drug is excreted completely by that particular organ.

The organ clearance can be determined from the organ blood flow and its extraction ratio ( CL_{organ = Qorgan . E).}

The organ clearance can be determined from the excretion rate over a short interval and the average plasma concentration during this interval. (CL_organ= Drug excretion rate by the organ/Cp_average).

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

The hepatic clearance cannot exceed the total body clearance. However it can be equal to the total body clearance when the drug is excreted completely by the liver.

The hepatic clearance can be determined from the liver blood flow and the hepatic extraction ratio CL_H = Q_liver . E

It is the maximum ability of the liver to eliminate the drug in absence of any flow limitation. It has units of volume/time. Based on this definition the hepatic clearance can be as high as the intrinsic clearance but it cannot be higher than it.

The hepatic intrinsic clearance is a measure of the amount of the enzymes available for drug metabolism. Enzyme induction increases the total amount of enzymes available for drug metabolism leading to higher hepatic intrinsic clearance. On the other hand liver diseases that will alter the ability of the liver to eliminate the drug decrease the hepatic intrinsic clearance.

For low extraction ratio drugs, the hepatic intrinsic clearance is close to the hepatic clearance, and for high extraction ratio drugs the intrinsic clearance is much higher than the hepatic clearance.

Enzyme induction means increasing the metabolic activity of the enzyme system. This can happen by a wide variety of agents known as enzyme inducers. Enzyme inducers usually increase enzyme synthesis leading to higher amount of the enzyme and increase in the capacity of one or more metabolic pathway.

The consequence of enzyme induction on drug metabolism depend on the drug. Some inducers are specific inducers of certain specific sub-family of CYP-450 which can affects the drugs metabolized by that particular CYP-450 sub-family.

Also drugs that are metabolized by high capacity metabolic pathways (high extraction ratio drugs, or drugs that have high intrinsic metabolic clearance) will not be affected significantly by enzyme inducers. On the other hand, enzyme induction significantly affects the rate of metabolism of drugs that have low intrinsic metabolic clearance (low extraction ratio drugs).

Competitive inhibitors of drug metabolism are compounds that can compete with the drug for the same metabolic pathway. Competitive inhibitors can be other drugs, drug metabolites, endogenous compounds or chemicals.

In absence of the inhibitor the drug is the only molecule metabolized by its specific enzyme system. In presence of the inhibitor, the inhibitor compete with the drug for the same enzyme system.

At any given drug concentration, the rate of drug metabolism is slower in presence of the inhibitor compared to when only the drug is present. The extent of inhibition depends on the drug concentration, the inhibitor concentration and the relative affinity of the drug and the inhibitor for the metabolizing enzyme.