GFR is measured by

Inulin is uniquely treated by nephrons in that it is completely filtered at the glomerulus but neither secreted nor reabsorbed by the tubules. This propey of inulin allows the clearance of inulin to be used clinically as a highly accurate measure of glomerular filtration rate (GFR) -- the rate of plasma from the afferent aeriole that is filtered into Bowman&;s capsule measured in mL/min.Ref: Ganong&;s review of medical physiology; 24th edition; page no; 679