Creatinine clearance is a method that estimates the glomerular filtration rate (GFR) of the kidneys.
Creatinine clearance is the amount of creatinine in the urine, divided by the concentration in the blood, over a certain amount of time.
Glomerular filtration rate can be calculated by measuring any chemical that has a steady level in the blood, and is filtered but neither actively absorbed or excreted by the kidneys. Creatinine is used because it fulfills these requirements (though not perfectly), and it is produced naturally by the body. Other methods involve constant infusions of inulin or another compound, to maintain a steady state in the blood.