Kidney function is defined by the glomerular filtration rate (GFR). It has been clearly established that serum creatinine should not be used alone to estimate GFR, and that estimated GFR should be calculated using formulas that take into account various parameters including serum creatinine. In adults, two formulas are mostly used to estimate GFR: the Cockcroft-Gault formula and the MDRD formula. In elderly, GFR should only be estimated using the MDRD formula, since the Cockcroft-Gault formula systematically underestimates GFR. GFR tends to decrease with age, but it should be 60 mL/min/1,73 m2 or higher in all subjects who do not suffer from kidney disease, regardless of age.