A new equation for estimating renal function using age, body weight and serum creatinine.

Background: Many formulas have been developed to estimate glomerular filtration rate (GFR). The aim of our study was to propose a new, more reliable equation.

Methods: The study considered 530 subjects (training sample) with M/F 280/250, age 57.1 +/- 17.4, creatinine clearance (CrCl) 55.2 +/- 38.2 (range 2.1-144.0) for the development the new equation. A linear model was used to describe Cr production using serum Cr (sCr), age, and body weight (BW) as variables: (CrCl + b(4)) . sCr = b(1) - (b(2) . age) + (b(3) . BW) subsequently estimating parameter values by linear least squares, with CrCl as the dependent variable, and 1/sCr, age/sCr, BW/sCr as independent variables. CrCl = {[69.4 - (0.59 . age) + (0.79 . BW)]/sCr} - 3.0 (males) and {[57.3 - (0.37 . age) + (0.51 . BW)]/sCr} - 2.9 (females). A 229-patient renal failure validation sample with M/F 166/63, age 53.0 +/- 14.8, GFR 32.0 +/- 14.3 (range 4.3-69.8), assessed using iohexol Cl, was considered to compare the Cockcroft-Gault (C-G) and MDRD formulas with the new equation for estimating GFR.

Results: The mean % error in GFR estimated by the new equation (+2.3 +/- 28.3%) was better than with the C-G and MDRD formulas (+5.2 +/- 30.1% and -11.4 +/- 25.9%, respectively, p < 0.0005 and p < 0.0001), and so was the mean absolute % error, bordering on statistical significance (19.8 +/- 20.3 vs. 21.1 +/- 22.0 and 22.4 +/- 17.3, p = 0.08 and p < 0.005). The precision was also better (RMSE = 7.89 vs. 8.02 and 9.13). The Bland-Altman test showed no GFR over or underestimation trend (measured +/- predicted GFR/2 vs. % error, R2 = 0.001).

Conclusions: The new equation appears to be at least as accurate as the C-G and MDRD formulas for estimating GFR.