Renal clearance is the amount of filtered blood that is “cleared” of a substance per unit time. It is calculated from the mass secreted per unit time divided by the plasma concentration of the substance. The mass secreted can be measured by multiplying urine volume by urine concentration. Clearance can be used to judge whether a substance has been filtered, reabsorbed, or secreted.
Inulin is a substance that is completely filtered and not reabsorbed. Thus, its clearance is equal to glomerular filtration rate (GFR). However, it is not a natural substance in the body, so it must be injected and is rarely used. Creatinine is a breakdown product of muscle phosphocreatine that has relatively constant plasma levels. It is also filtered and not reabsorbed, but a small amount is secreted. Creatinine clearance is commonly used to estimate GFR.
Glucose clearance is zero. It is filtered and completely reabsorbed. Partly reabsorbed substances have clearances between 0 and GFR. That is, there is less substance in the urine than was filtered from the blood. Secreted substances have higher clearance than GFR (more substance in the urine than was filtered).
Making a Difference in the World 