What is the substance used to measure the glomerular filtration rate?

Inulin is a substance foreign to the body that, when administered at a certain concentration in the blood, has a clearance of 125 mL/min.
The clearance value for inulin can be used to measure the glomerular filtration rate (GFR).
The clearance value for inulin is the same as the volume of glomerular filtration.
The clearance value for inulin can be used to measure the renal plasma flow (RPF).