To measure the glomerular filtration rate (GFR), the clearance of substances is used, which in the course of transport through the kidneys are only filtered without undergoing reabsorption or secretion in the tubules, dissolve well in water, freely pass through the pores of the basal glomerulus membrane and do not bind to plasma proteins. Such substances include inulin, endogenous and exogenous creatinine, urea.