Currently, the most widely-utilized method for routine estimation of GFR (eGFR) is the 4-variable MDRD Equation, in which serum creatinine, age, gender and race are the variables entered into a mathematical formula. eGFR as determined by the MDRD Equation also serves as the basis for CKD Staging. Despite the fact that MDRD appears to be relatively successful as estimating an accurate GFR for patients with CKD Stage 3 and above, there are some increasingly recognized limitations to its use. Most notably, the MDRD Equation is not as accurate for higher degrees of GFR (e.g., GFR > 60 cc/min), and overall tends to overestimate the prevelance of CKD stage 1 & 2 in the general population. In addition, the original MDRD cohort did not contain any patients with diabetic kidney disease or those > 70 years of age, both of which represent a sizeable (and growing) portion of today's CKD population.
To help deal with these limitations, researchers have developed a modified version of the MDRD called the CKD-EPI Equation, recently published by Levey et al in a 2009 issue of Annals of Internal Medicine. Briefly, researchers took a larger and more diverse patient population and measured their GFR using isothalamate clearance as a "gold standard." They then performed a series of statistical manipulations to generate a slightly different mathematical equation than the MDRD in order to arrive at an estimated GFR using the 4 standard values. In particular, they used a mathematical function termed a "spline-knot function" (way beyond my field of expertise) which apparently reduces bias at higher levels of kidney function. Perhaps this equation, with the appropriate follow-up tests & validations, could one day replace the MDRD Equation in routine clinical use, and be more useful at estimating kidney function in individuals with less severe disease.