In normal conditions only 75% of kidney function capability are in use. Resting part of kidney function is called renal functional reserve. It is a very important indicator for prediction of renal insufficiency in chronic renal diseases, morphological changes or reduced mass of kidney parenchyma. More common test to estimate renal functional reserve is a method of measurement of glomerular filtration rate (clearance) before and after provocation of renal filtration by acute peroral protein load. The value and the possibilities to put in practice this test as well the factors influencing the results of glomerular filtration rate were estimated in this review.