New Predictive Equations Improve Monitoring of Kidney Function in Patients with Diabetes

Abstract

Objective-- The Cockcroft-Gault (CG) and MDRD equations poorly predict GFR decline in diabetic patients. We sought whether new equations based on serum creatinine (Mayo Clinic Quadratic -MCQ- or re-expressed MDRD -rMDRD) or 4 cystatin C--based equations (Cys-eGFR) were less biased and better predicted GFR changes.

Research Design and Methods-- In 124 diabetic patients with a large range of isotopic GFR (56.1^±35.3 mL/min/1.73m², range 5-164), we compared the performances of the equations before and after categorization in GFR tertiles. Twenty patients had a second determination two years later.

Results-- The CG was the least precise. The MDRD was the most precise but the most biased according to the Bland & Altman procedure. By contrast with the MDRD and, to a lesser extent, MCQ, three of the four Cys-eGFR were not biased. All equations overestimated the low GFRs, whereas only the MDRD and Rule's equation underestimated the high GFRs. For the subjects studied twice, isotopic GFR changed by -8.5±17.9 mL/min/1.73m². GFR changes estimated by the CG (-4.5±6.8) and MDRD (-5.7±6.2) did not correlate with the isotopic changes, whereas new equations-predicted changes did: MCQ: -8.7±9.4; (r=0.44, P<0.05) and all four Cys-eGFR: -6.2±7.4 to -7.3±8.4 (r =0.60 to 0.62, all P<0.005), such as 100/cystatin C (r=0.61, P<0.005).

Conclusions-- The new predictive equations better estimate GFR than the CG. Although the MDRD remains the most accurate, it poorly predicts GFR decline as it overestimates low and underestimates high GFR. This bias is lesser with the MCQ and Cys-eGFR, so they better predict GFR changes.