Clearance studies are widely used to assess glomerular filtration rate and renal blood flow and to study the excretion of various substances by the kidney. The renal clearance of a substance represents the virtual or theoretical volume of plasma which is completely cleared of this substance in a given unit of time. It is usually expressed in milliliters per minute and calculated from the formula UV÷P, where U and P represent the urine and plasma concentration and V is the volume of the urine.1-3 Clearances have played an important role in the study of renal physiology and have provided much circumstantial evidence regarding the relative importance of glomerular and tubular excretory mechanisms.
Since retention of urea in the blood is accompanied by decreased urinary excretion, students of renal disease in the early 1900's had attempted to evaluate renal function by relating the concentration of urea in urine
Dunea G, Freedman P. Renal Clearance Studies. JAMA. 1968;205(3):170-171. doi:10.1001/jama.1968.03140290062018