A New Equation for Estimating Renal Function Using Age, Body Weight and Serum Creatinine

Abstract

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 + b₄) · sCr = b₁ – (b₂ · age) + (b₃ · 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.