Monitoring kidney function in type 2 diabetic patients with incipient and overt diabetic nephropathy

OBJECTIVE: The purpose of this study was to assess agreement between glomerular filtration rate (GFR) and the decline in GFR estimated with the Modification of Diet in Renal Disease (MDRD) Study Group equation or the Cockcroft-Gault formula and measured by the plasma clearance of 51Cr-EDTA. RESEARCH DESIGN AND METHODS: We followed a cohort of 156 microalbuminuric type 2 diabetic patients for 8 years with four measurements of GFR and another cohort of 227 type 2 diabetic patients with overt diabetic nephropathy for 6.5 (range 3-17) years with seven (3-22) measurements of GFR. RESULTS: For patients with microalbuminuria, mean +/- SD baseline GFR was 117 +/- 24 measured, 92 +/- 20 estimated (MDRD equation), or 103 +/- 24 ml x min(-1) per 1.73 m2 estimated (Cockcroft-Gault formula) (both P < 0.001); 95% limits of agreement were -66.1 to 20.3 (MDRD equation) and -58.7 to 30.7 (Cockcroft-Gault formula). The rate of decline in GFR was 4.1 +/- 4.2 measured, 2.9 +/- 2.8 estimated (MDRD equation), or 3.4 +/- 3.2 ml x min(-1) per 1.73 m2 estimated (Cockcroft-Gault formula) (both P < 0.001). For patients with overt nephropathy, baseline GFR was 84 +/- 30 measured, 73 +/- 24 estimated (MDRD equation), or 81 +/- 28 ml x min(-1) per 1.73 m2 estimated (Cockcroft-Gault formula) (both P < 0.001) with 95% limits of agreement -47 to 25 (MDRD equation) and -39 to 33 (Cockcroft-Gault formula). The rate of decline in GFR was 5.2 +/- 4.1 measured, 4.2 +/- 3.8 estimated (MDRD equation), and 4.6 +/- 4.1 ml x min(-1) per 1.73 m2 estimated (Cockcroft-Gault formula) (both P < 0.001). CONCLUSIONS: Particularly in microalbuminuric (hyperfiltering) patients, GFR is significantly underestimated with wide limits of agreement by the MDRD equation as well as by the Cockcroft-Gault formula. The rate of decline in GFR is also significantly underestimated with both equations. This makes GFR estimations based upon these equations unacceptable for monitoring kidney function in type 2 diabetic patients with incipient and overt diabetic nephropathy.