Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal glomerular capillaries into Bowman's capsule per unit time. Clinically, this is often measured to determine renal function.

The creatinine clearance was originally determined by injecting inulin (not insulin) into the plasma. Since inulin is not absorbed into any body systems, it is 100% filtered by the glomerulus and was represented with (volume of plasma)/(time). However, today we use creatinine clearance in order to know the filtration rate. Reasons for this are because it is like inulin and not reabsorbed from the renal tubules and is also synthesized by the body, making it a much cheaper yet still effective way to obtain clearance rates. The rate is typically recorded in 100ml/min.

Example: A patient having a 0.1mg/ml blood concentration of inulin can have his GFR shown by 0.1mg/ml x 100ml/min = 10mg/min. From this, the patient has a clearance rate of 10mg of inulin in one minute so a 20mg injection would be cleared in 2 minutes.