Since gold-standard measurements of glomerular filtration rate (GFR) are impractical, accurate estimated GFR (eGFR) based on endogenous biomarkers is essential.1 The 2009 Chronic Kidney Diseases Epidemiology Collaboration (CKD-EPI) creatinine equation does not seem to perform as well in adolescents and young adults (AYA).2 In 2021 Pierce et al3 developed eGFR equations (“U25 equations”) using either cystatin C or serum creatinine for children/AYA up to 25 years old with mild-to-moderate chronic kidney disease enrolled in the CKiD study.