In the United States, glomerular filtration rate (GFR) is commonly estimated using serum creatinine and the 2021 Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation for individuals older than 18 years or the 2021 Chronic Kidney Disease in Children Under 25 Study (CKiD-U25) equation for those between 1 and 25 years of age with CKD (Item S1).1,2 These equations may result in different estimated GFR (eGFR) values at 18 years and older, leading to uncertainty in assessment of severity of disease, progression rate, and clinical decisions based on level of GFR.