Clearance is the rate of elimination of substances from the blood. Systemic or total body clearance (Cltot) is calculated as the rate of elimination divided by serum concentration, where elimination can occur by both metabolic and renal clearance mechanisms.
Renal clearance (Clr) is the total amount of drug excreted over a specific time and will depend on glomerular filtration, tubular secretion and tubular reabsorption.
All drugs have different clearance rates, and this is important to know for determining the right dosage. Some have a high clearance, meaning that they are eliminated from the blood rapidly by the kidneys, such as the diuretic frusemide. Others have a low clearance due to inefficient excretion, so only low levels need to be administered to maintain their therapeutic level in the blood.