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.