GFR stands for the glomerular filtration rate. The kidneys work by filtering the blood through tiny filters; there are several hundred thousand in each kidney (364,161 to 586,095) - Image Anal Stereol 2000; 20:105-108 - in each kidney. These filters are called glomeruli, and the rate at which they filter waste products is an index of how well they function. Hence, it is a good indicator of the presence of chronic kidney disease (CKD)
The MDRD Study (Modification of Diet in Renal Disease) was designed to look at the effect of dietary protein restriction on the prevention of kidney disease, and required very accurate measurements of the GFR. These measurements, while difficult and expensive, could be mathematically correlated with age, race, gender and the serum creatinine. Creatinine is a waste product of muscle breakdown, and an excellent marker for use in determining kidney function.
The MDRD study equation, when expressed mathematically, could be easily programmed and used for the classification of kidney disease. It was discovered that by combining the results from several other large trials, one could get an epidemiologically more accurate measure of the GFR. The equation developed in 2009 is known as the CKD-EPI Creatinine equation. It is the one recommended for everyday use.
These equations are available on the web at ckd-epi.com.