Significant differences when using MDRD for GFR estimation compared to radionuclide measured clearance

Objectives: To evaluate Estimated Glomerular Filtration Rate (eGFR), using the Modification of Diet in Renal Disease equation, and compare with radionuclide GFR (rGFR) in a Radiology setting to assess renal function prior to contrast administration. Methods: Five hundred and sixteen retrospective rGFR studies from a mixed referral population were selected and the eGFR calculated. Regression and Bland-Altman analysis was performed. The percentage of rGFR and eGFR studies below 30 ml/min/1.73 m ² and 60 ml/min/1.73 m ² were calculated; these are important thresholds for classifying renal insufficiency. Results: A significant correlation between eGFR and rGFR (R ²=0.62, p<0.0001) and significant differences in the medians (p<0.0001) were found. eGFR overestimated rGFR with a bias (mean difference) of 10.8 ml/min/1.73 m ² over the whole range of rGFR. Studies with an rGFR of under 30 ml/min/1.73 m ² had a mean bias of 4.6 ml/min/1.73 m ² (difference range -5.9 to 26.3 ml/min/1.73 m ²). The bias over the range 30 to 60 ml/min/1.73 m ² was 13.2 ml/min/1.73 m ² (difference range -16.8 to 88.3 ml/min/1.73 m ²). In 25.4% of studies, eGFR was less than 60 ml/min/1.73 m ² compared with 40.5% of rGFR studies. Conclusions: Awareness of the bias between eGFR and rGFR is important when assessing Radiology patients for risks of nephrotoxicity and Nephrogenic Systemic Fibrosis from contrast medium.