Supplementary Material for: Comparison between old and new GFR estimating equations in children and adults with glomerular disease in the NEPTUNE study

Introduction: New equations developed in the US for estimating glomerular filtration rate (GFR) eliminated race for adults and widened the age range for children and young adults. The European Kidney Function Consortium (EKFC) equation was also validated and updated for a US adult population. The aftereffects of adopting these new equations on previous research results among patients with glomerular disease are unknown. This study compared eGFR using old and new estimating equations and their impacts on eGFR-based outcomes. Methods: Longitudinal serum creatinine measurements from children and adults enrolled in the Nephrotic Syndrome Study Network (NEPTUNE) were used to calculate eGFR using old Bedside Schwartz and CKD-EPI 2009 equations, new U25 and race-free CKD-EPI 2021 equations, and the EKFC equation. Time to disease progression (40% eGFR decline or kidney failure) outcomes were compared using Kaplan-Meier curves and Cox models and longitudinal eGFR outcomes were compared using linear mixed effects models to assess effects of demographics, clinical characteristics, pathology descriptors, a serum and urine biomarker, and the APOL1 genetic trait. Results: N=756 NEPTUNE study participants were included (median age 21 years, 41% female, and 25% who reported Black race). Disease progression outcomes were similar between using old vs. new age-specific equations, whereas event rates were lower using EKFC. Survival curves were largely overlapping, and selected risk factor effects on disease progression were similar. Only sex and race effects on longitudinal eGFR differed between old vs. new age-specific equations, whereas larger differences were observed for disease diagnosis effects when using EKFC. Conclusion: New U25 and race-free CKD-EPI 2021 equations had little impact on estimated GFR values and results of survival and longitudinal regression analyses. EKFC results differed and was likely driven by those with very high eGFR.