Evidence of the errors of estimated GFR in patients with diabetes
The utility of eGFR in type 2 diabetes is still unclear. Several studies evaluated the agreement between eGFR and mGFR in this group of patients yielding contradictory results (1). Luis-Lima et al. analyzed a large cohort with different underlying renal diseases, normo-, micro- and macroalbuminuria, and mGFR ranging from 8.5 to 180.6 mL/min. The authors also estimated GFR with 61 different formulas based on creatinine and/or cystatin-C levels. The error of eGFR calculated with any equation was common and random, averaged 40% of real renal function, and was larger in patients with mGFR below 60 mL/min. In addition, nearly 30% of the individuals were misclassified in chronic renal disease stages, and 25% of those with hyperfiltration were not diagnosed (1). Thus, the currently available formula to estimate GFR may be considered unreliable in type 2 diabetes.
Besides the actual GFR value, its decline also cannot be accurately estimated in type 2 diabetes (4). In a large study involving 600 patients with type 2 diabetes, Gaspari et al. evaluated the agreement between GFR measured by different methods at baseline, month 6, and long-term follow-up. The results confirmed that both the simplified MDRD equation and the CKD-EPI formula underestimate mGFR in diabetic subjects, and that this limitation must be extended to other 13 commonly used formulas implemented over the past fifty years (4). The most probable cause for these discrepancies is the fact that creatinine and cystatin-C are not accurate enough markers of renal function.
What can we do to improve GFR assessment in everyday practice?
Whenever the clinical situation requires meticulous evaluation of renal function mGFR should be obtained. As an example of good practice, at the University of La Laguna on Tenerife the Laboratory of Renal Function was founded to validate the plasma clearance of iohexol for clinical purposes (http://lfr.ecifucan.es). The method used to evaluate plasma clearance of iohexol is dry blood spot (DBS) sampling, which proved to be cheap, safe, and simple, not requiring any special equipment, such as freezers, tubes, or centrifuge (Figure 3). The test costs 50€ and gives an accurate assessment of renal function.