Are prediction equations for glomerular filtration rate useful for the long-term monitoring of type 2 diabetic patients?

Background. The aim of this study was to compare the accuracy of prediction equations [modification of diet in renal disease (MDRD), simplified MDRD, Cockcroft–Gault (CG), reciprocal of creatinine and creatinine clearance] in a cohort of patients with type 2 diabetes. Methods. A total of 525 glomerular filtration rates (GFRs) using 125I-iothalamate were carried out over 10 years in 87 type 2 diabetic patients. Accuracy was evaluated at three levels of renal function according to the baseline values obtained with the isotopic method: hyperfiltration (GFR: >140 ml/min/1.73 m2; 140 isotopic determinations in 27 patients), normal renal function (GFR: 140–90 ml/min/1.73 m2; 294 isotopic determinations in 47 patients) and chronic kidney disease (CKD) stages 2–3 (GFR: 30–89 ml/min/1.73 m2; 87 isotopic determinations in 13 patients). The annual slope for GFR (change in GFR expressed as ml/min/year) was considered to ascertain the variability in the equations compared with the isotopic method during follow-up. Student's t-test was used to determine the existence of significant differences between prediction equations and the isotopic method (P < 0.05 with Bonferroni adjusted for five contrast tests). Results. In the subgroup of patients with hyperfiltration, a GFR slope calculated with 125I-iothalamate −4.8 ± 4.7 ml/min/year was obtained. GFR slope in patients with normal renal function was −3.0 ± 2.3 ml/min/year. In both situations, all equations presented a significant underestimation compared with the isotopic GFR (P < 0.01; P < 0.05). In the subgroup of CKD stages 2–3, the slope for GFR with 125I-iothalamate was −1.4 ± 1.8 ml/min/year. The best prediction equation compared with the isotopic method proved to be MDRD with a slope for GFR of −1.4 ± 1.3 ml/min/year (P: NS) compared with the CG formula −1.0 ± 0.9 ml/min/year (P: NS). Creatinine clearance presented the greatest variability in estimation (P < 0.001). Conclusions. In the normal renal function and hyperfiltration groups, none of the prediction equations demonstrated acceptable accuracy owing to excessive underestimation of renal function. In CKD stages 2–3, with mean serum creatinine ≥133 µmol/l (1.5 mg/dl), the MDRD equation can be used to estimate GFR during the monitoring and follow-up of patients with type 2 diabetes receiving insulin, anti-diabetic drugs or both.