To directly measure GFR, renal clearance of an infused compound is measured in urine. For these studies, molecular markers are selected where their appearance in the urine is virtually equal to the amount that crosses the glomerulus.
The gold standard marker for direct GFR measurement is Inulin. However, it is not commonly employed in the United States due to limitations in solubility and difficulty in measurement compared to other compounds.
More common markers include Iohexol and Iothalamate. Both can be measured by HPLC and are generally safe for use.
