GFR-estimating models in kidney transplant recipients on a steroid-free regimen

How to best estimate glomerular filtration rate (GFR) in kidney transplant recipients on steroid-free immunosuppression has not been established.

Within 3 months of transplantation, iothalamate GFR (iGFR) was measured in 107 recipients on steroid-free and 27 on steroid-maintenance immunosuppression. A year later, a second GFR was performed. Serum creatinine was calibrated against a reference laboratory, and GFR was estimated (eGFR) using the re-expressed Cockcroft-Gault equation, eGFRCG; the Mayo Clinic equation, eGFRMC; the Modification of Diet in Renal Disease (MDRD) study equation, eGFRMDRD; and the newly introduced Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation.

All models overestimated GFR regardless of steroid use or timing of GFR. In those not receiving steroids, eGFRCG was least biased: 1.85 +/- 15.2 ml/min at the first GFR and 0.23 +/- 15.2 ml/min at the second. eGFRMC and eGFRCKD-EPI were most biased and were within 30% of iGFR less than 60% of the time in contrast to eGFRCG which was within 30% of iGFR 80.2% of the time. eGFRMDRD was intermediate in its performance at the first GFR but was comparable to eGFRCG at the second measurement. Importantly, the four models had comparable but poor precision. Exposure to steroids for a whole year did not appreciably alter the models' bias or relative accuracy but resulted in a dramatic fall in their precision, R2 = 0.05-0.12.

GFR prediction equations overestimate measured GFR in recipients on and off steroid regimens. Long-term exposure to steroids results in a marked reduction in the precision of all models. In all, eGFRCG and eGFRMDRD are the two best available models.