The practical implications of using estimated GFR as the presumed reference variable to estimate transplant chronic kidney disease

We determined the proportions of matched kidney transplant isotope GFRs (iGFRs) to the estimated functions (eGFRs) calculated from Isotope Dilution Mass Spectrometry (IDMS), Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI), and Cockcroft-Gault (CG) equations. Method: 1403 iGFR/eGFR pairs on 390 kidney transplant patients were compared considering the iGFR or eGFR as the reference or test variable. Results: Conformity of iGFR to CG estimates demonstrated the least bias of 1.3±18.4 mL/ min/1.73 m2 (compared to 1.5±19.4 and -2.2±19.2 for IDMS and CKDI-EPI, P<.05) and CKD-EPI estimates the highest precision of 4.1±41.8 (compared to 11.3±43.9 for IDMS and 5.7±37.3 for CG; P<.05). IDMS eGFR cut-off <60 and <30 mL/min/1.73m2 were correctly matched by iGFR in 79.4% and 49.1% of the times, while CKD-EPI was matched by iGFR in 83.5% and 52.5%. CG was matched in 78.3% and 53.6%. IGFR cut-off levels of <60, and <30 mL/min/1.73m2 were predicted by IDMS in 83.8 % and 64.0% of the times. CKD-EPI was correct in 77.8% and 59.0% and CG in 82.5% and 41.6% respectively. Conclusion: Transplant eGFR results obtained by CKD-EPI or CG are likely to be more precise and less biased than IDMS.