Final answer:
Clearance refers to the volume of blood cleared of a substance per time, not serum concentration. Urea and creatinine clearance tests are examples of how clearance is measured, with creatinine clearance providing an estimate of the glomerular filtration rate (GFR). Maximal clearance occurs when elimination rate is directly proportional to plasma concentration.
Step-by-step explanation:
Clearance (CL) is the theoretical volume of blood that is cleared completely of a substance per unit of time. It is not related to the amount of substance actually present in the blood (serum concentration) but is a measure of how effectively the blood is cleared of that substance.
The urea clearance test, for instance, calculates the volume of plasma that is cleared of urea per minute based on urine flow (V) and the concentration of urea in the plasma ('B').
The creatinine clearance test measures the glomerular filtration rate (GFR), which estimates the volume of fluid filtered from kidney glomerular capillaries into Bowman's capsule per unit time. The formula for creatinine clearance (CC) is CC = (U x V) / B. Creatinine clearance can be corrected for body surface area (BSA) to allow comparison between individuals of different sizes.
In a situation called maximal clearance, when urine volumes are large, the rate of elimination of a substance is directly proportional to its plasma concentration. Maximal clearance tests typically involve excretion rates of 2 ml or more of urine per minute.