Renal clearance of substance X (Cx) is the volume of plasma completely cleared of that substance by the kidneys per unit time: Cx = (Ux × V) / Px.
Ux is the urinary concentration of X, V is the urine flow rate per unit time, and Px is the plasma concentration of X.
Criteria for an Ideal Clearance Substance
A substance used to measure glomerular filtration rate (GFR) by clearance must be: freely filtered by the glomeruli, and neither reabsorbed from nor secreted into the tubules.
It must also not be synthesized, stored or altered in the kidney, not metabolized in the body, non-toxic, and easily measurable in plasma and urine.
Inulin, a polymer of fructose, satisfies all these criteria and is the reference substance for GFR measurement.