Clearance

Clearance

Clearance is a pharmacokinetic measurement of the volume of plasma from which a substance is completely removed per unit time. It is a crucial concept in understanding how drugs are processed in the body and is used to determine appropriate dosing regimens for medications.

Definition

Clearance is defined as the rate at which a substance is removed from the body divided by the concentration of the substance in the plasma. It is usually expressed in units of volume per time, such as milliliters per minute (mL/min) or liters per hour (L/h).

Types of Clearance

There are several types of clearance, each relevant to different physiological processes:

• Renal Clearance: This refers to the volume of plasma that is cleared of a substance by the kidneys per unit time. It is a key parameter in assessing kidney function and is often measured using substances like creatinine or inulin.

• Hepatic Clearance: This is the volume of plasma cleared of a substance by the liver. It involves processes such as metabolism and biliary excretion.

• Total Body Clearance: This is the sum of all clearance processes in the body, including renal, hepatic, and other routes of elimination.

Calculation of Clearance

Clearance can be calculated using the formula:

\[ \text{Clearance} (Cl) = \frac{\text{Rate of Elimination}}{\text{Plasma Concentration}} \]

For drugs that follow first-order kinetics, the rate of elimination is proportional to the concentration of the drug in the plasma.

Clinical Significance

Understanding clearance is essential for determining the correct dosage of a drug. It helps in:

• Dosing Regimens: Adjusting doses to achieve therapeutic levels without causing toxicity.
• Drug Interactions: Predicting how one drug might affect the clearance of another.
• Assessing Organ Function: Evaluating kidney and liver function based on how well they clear substances from the body.

Factors Affecting Clearance

Several factors can influence clearance, including:

• Age: Clearance can decrease with age due to reduced organ function.
• Genetics: Genetic variations can affect enzyme activity involved in drug metabolism.
• Disease States: Conditions like liver or kidney disease can significantly alter clearance.
• Drug Interactions: Some drugs can inhibit or induce enzymes that affect clearance.

Also see

• Pharmacokinetics
• Renal function
• Liver function tests
• Drug metabolism