The CKiD creatinine- and cystatin C–based glomerular filtration rate (GFR) estimating equations have recently been modified by incorporation of continuous age- and sex-dependent k values to yield less biased internal validation results in chronic kidney disease (CKD) patients aged under 25 years.1 Here we report an external validation of these CKiDU25 equations in the European Kidney Function Consortium (EKFC) multicenter cohort (Table 1, Item S1).2,3 In this cohort of children and young adults, the vast majority have a measured GFR (mGFR) ≥75 mL/min/1.73 m2, thus resembling a setting where CKD is screened for.