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What happens when an 18-year-old patient’s kidney function appears to change overnight—not because their kidneys changed, but because the laboratory switched eGFR equations? This scenario is more common than many laboratorians realize and highlights an ongoing challenge in kidney function reporting for adolescents and young adults. While adult patients are typically assessed using CKD-EPI equations, the transition from pediatric to adult kidney function estimation is not always straightforward. In fact, switching equations at age 18 can sometimes create an apparent change in kidney function even when the patient's renal status has not changed at all.
Historically, many laboratories reported creatinine-based eGFR only for adult patients. Pediatric nephrologists often relied on equations such as the bedside Schwartz equation, which was developed specifically for children with chronic kidney disease. Although widely adopted, the bedside Schwartz equation has limitations. Studies have demonstrated an age-related decline in eGFR estimates even among healthy children and adolescents, raising concerns that part of the observed change may reflect characteristics of the equation itself rather than true physiologic differences. To address this issue, investigators from the Chronic Kidney Disease in Children (CKiD) study developed the CKiD Under 25 (U25) equations. By incorporating age- and sex-specific coefficients, these equations were designed to provide a smoother transition from childhood through young adulthood.
This question recently became particularly relevant at our institution as we evaluated how eGFR should be reported across different age groups. Previously, creatinine-based eGFR was reported primarily using CKD-EPI equations in adults, while cystatin C testing for pediatric patients utilized the CKiD U25 framework. More recently, we implemented a standardized age-based approach in which patients aged 1 to <25 years receive eGFR calculations using CKiD U25 equations, while patients aged ≥25 years receive CKD-EPI-based estimates. This change was motivated by growing evidence that pediatric and adult creatinine-based equations do not fully converge during late adolescence and early adulthood. As a result, CKD stage classification may differ simply because a different equation is applied rather than because kidney function has truly changed.
The conversation does not end with creatinine alone. Increasing evidence suggests that combining creatinine and cystatin C improves eGFR accuracy in many pediatric populations, including transplant recipients and oncology patients. As more laboratories adopt cystatin C testing, the opportunity to implement combined creatinine-cystatin C U25 equations becomes increasingly attractive. While no single equation performs best in every clinical setting, the U25 framework offers a practical solution for one of the most challenging periods in kidney function assessment-the transition from pediatric to adult care. As more laboratories adopt cystatin C testing and reevaluate eGFR reporting practices, an important question remains: should age 18 continue to define the transition to adult equations, or is age 25 a more physiologically appropriate boundary?
References
- Pottel, Hans, Jonas Björk, Pierre Delanaye, and Ulf Nyman. "Evaluation of the creatinine-based chronic kidney disease in children (under 25 years) equation in healthy children and adolescents." Pediatric Nephrology 37, no. 9 (2022): 2213-2216.
- Filler, Guido, Fateh Ahmad, Vipin Bhayana, Maria E. Díaz González de Ferris, and Ajay P. Sharma. "Limitations of U25 CKiD and CKD-EPI eGFR formulae in patients 2–20 years of age with measured GFR> 60 mL/min/1.73 m2—a cross-sectional study." Pediatric Nephrology 39, no. 4 (2024): 1169-1176.
