The glomerular filtration rate (GFR) is the amount of blood which is completely cleared of a substance which is freely filtered by the kidneys, per unit time. The normal units are mls/min and the value is then corrected for body surface area and described as mls/min/1.73m2.
Creatinine is used as a measure of GFR in that it is freely filtered, although it is also secreted into the urine through the proximal tubule and therefore at a low GFR, creatinine clearance will over-estimate GFR.
Normal values for GFR are > 80 mls/min/1.73m2.