[Rationale, pros and cons of GFR estimation: the Cockcroft-Gault and MDRD equations]

Current guidelines suggest to evaluate kidney function by estimates of the glomerular filtration rate (GFR) calculated using the Cockcroft-Gault equation or the simplified equation of the Modification of Diet in Renal Disease (MDRD) study. Cockcroft and Gault developed a method to predict an individual's creatinine clearance without urinary data but on the basis of average creatinine clearance values corrected for age and body weight. The equation needs information about gender, age, weight, and serum creatinine. The precision of the equation is biased by overweight because the equation is based on the assumption that creatinine clearance linearly reflects body weight. The MDRD equation derives from a multiple regression used to analyze the relation of GFR measurements by a radioisotope technique over serum creatinine after data linearization by logarithm transformation and with control for gender, age, ethnicity and other variables. The equation has not been validated for GFR >60 mL/min x 1.73 m2 because the study did not include healthy persons. The two equations often give conflicting estimates of GFR. Nephrologists have to understand the rationale of the two equations for the correct interpretation of these discrepancies.