Low agreement among the equations in healthy children and young adults.

BACKGROUND: Several updated equations based on creatinine and/or cystatin C have been developed to estimate glomerular filtration rate (GFR), but their concordance in adolescents and young adults remains unclear. The CKiD Under 25 years (CKiDU25) equation was developed in children and young adults with chronic kidney disease, whereas the European Kidney Function Consortium (EKFC) equation was designed for use across the lifespan. Their agreement and the factors associated with eGFR category reclassification in healthy populations remain unexplored. METHODS: We compared the CKiDU25 and EKFC equations using combined estimates, defined as the mean of creatinine- and cystatin C-based eGFR for each equation, in adolescents and young adults from the National Health and Nutrition Examination Surveys 1999-2002. Factors associated with reclassification by the EKFC equation were examined. RESULTS: The study included 4552 adolescents and young adults, representing a weighted population of approximately 57.26 million. The CKiDU25 equation showed an age-related decline in eGFR, particularly among Black males, whereas EKFC yielded more consistent eGFRs across demographic groups. Compared with CKiDU25, EKFC markedly reduced the estimated prevalence of reduced kidney function, defined as eGFR 60-89 mL/min/1.73 m2, in the healthy population (4.06% vs. 14.16%). Overall, 66.74% of participants were reclassified to a lower eGFR category by EKFC. Lower reclassification was more common among older participants, females, those without diabetes, and those with higher body mass index and waist-to-height ratio. CONCLUSIONS: Equation selection substantially affects eGFR estimation and classification in healthy adolescents and young adults, underscoring the need to consider age, sex, and race when choosing GFR estimation equations.