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)
Creatinine is a waste product of muscle breakdown. It is completely filtered by the glomeruli and is therefore an excellent marker for use in determining kidney function. However, the normal levels in the blood differ by age, sex and race.
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 are difficult and expensive and cannot be used in routine clinical practice. The investigators developed an equation to estimate the GFR, by mathematically adjusting the serum creatinine for age, race, and gender.
The MDRD study equation, when expressed mathematically, could be easily programmed and used for the classification of kidney disease. However, it is limited as was developed only in patients with CKD and therefore underestimates measured GFR at levels higher than 60. The Chronic Kidney Disease-Epidemiology Collaboration (CKD-EPI), by a collaborative research team lead by Andrew Levey and Lesley (Stevens) Inker, combining the results from multiple other large studies that included people with and without CKD, and who had a wide range of age and GFR. Using this larger dataset, they developed a more accurate equation. The equation developed in 2009 is known as the CKD-EPI Creatinine equation. (Ann Intern Med. 5 May 2009;150(9):604-612) Recently this equation has also been shown to be a better predictor of adverse outcomes than the MDRD Study equations. (JAMA. 2012 May 9;307(18):1941-51) It is the one recommended for everyday use.
Sometimes muscle mass may vary, and another marker, cystatin C, can be used in its place, or in combination with the creatinine. The CKD-EPI investigators developed the Cystatin C equation and the CKD-EPI Cystatin C-Creatinine equations in 2012 N Engl J Med. 2012 Jul 5;367(1):20-9. The four calculators referenced above were converted to a web application by this author in cooperation with Drs. Levey and Inker, and can be found at ckd-epi.com.
Stephen Fadem and Leslie (Stevens) Inker