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

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.

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 m² and 60 ml/min/1.73 m² were calculated; these are important thresholds for classifying renal insufficiency.

A significant correlation between eGFR and rGFR (R² = 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 m² over the whole range of rGFR. Studies with an rGFR of under 30 ml/min/1.73 m² had a mean bias of 4.6 ml/min/1.73 m² (difference range -5.9 to 26.3 ml/min/1.73 m²). The bias over the range 30 to 60 ml/min/1.73 m² was 13.2 ml/min/1.73 m² (difference range -16.8 to 88.3 ml/min/1.73 m²). In 25.4% of studies, eGFR was less than 60 ml/min/1.73 m² compared with 40.5% of rGFR studies.

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.