The accuracy of estimating the glomerular filtration rate (GFR) on the basis of the serum creatinine concentration is limited because the serum creatinine concentration is affected by factors other than creatinine filtration. In an attempt to circumvent this limitation, formulas have been developed that include age, sex and body size in the calculation. The most widely used formula is the Cockcroft–Gault calculation of creatinine clearance. Levey and associates, as part of the Modification of Diet in Renal Disease (MDRD) study, developed an equation that may serve as an improvement in the method of predicting GFR on the basis of the serum creatinine concentration.
The authors used data from the MDRD study to devise a more accurate method of estimating GFR. Of the 1,785 patients who entered the study at baseline, 1,628 underwent measurement of GFR and other variables. The GFR was measured as the renal clearance of 125I-iothalamate. Creatinine clearance was computed from a 24-hour urine collection and a single measurement of serum creatinine.
The accompanying table on page 3212 shows the seven equations evaluated as methods of estimating the GFR based on the serum creatinine concentration. The equations that were based on measured or estimated creatinine clearance were found to overestimate GFR. Inclusion of factors such as age, sex and ethnicity improved the accuracy of estimations. The MDRD study prediction equation—equation 7 in the table—predicted the GFR more accurately than the other formulas. This equation accounts for the variability in urea and creatinine filtration by the renal tubules and for the phenomenon of a reduction in GFR that occurs with lower protein intake.
The authors conclude that this new equation for estimating GFR could be implemented by clinical laboratories, provided demographic characteristics (age, sex and ethnicity) are available at the time serum measurements are taken.