Estimating GFR in Young Adults with Cystatin C

Background: Several research groups have developed eGFR equations for GFR evaluation in young adults. The use of different equations may lead to different eGFR values and uncertainty in GFR-based decision making. We previously showed that creatinine-based eGFR (eGFRcr) using the CKD-EPI 2021 had a small overestimate of measured GFR (mGFR), whereas European Kidney Function Consortium (EKFC) and Children under 25 (CKiD U25) equations had a small to moderate underestimate. We hypothesized that the differences among equation performance were related to populations differences in non-GFR determinants of serum creatinine, such as muscle mass, in the development dataset. Cystatin C is less affected by these non-GFR determinants and therefore cystatin C based eGFR (eGFRcys and eGFRcr-cys) equations may be more similar across populations. Methods: We compared the CKD-EPI, EKFC and CKiD U25 eGFR equations in young adults (aged 18 to 40 years) in an independent dataset (751 participants from 9 studies). Equation performance compared to measured GFR was assessed as bias (median difference between mGFR and eGFR) and percentage of eGFR within 30% of mGFR (P30). Results: Mean (SD) age was 31.9 (5.8) years and mGFR was 96.1 (32.2) mL/min/1.73m2. There was small bias for the CKD-EPI and EKFC eGFRcys and eGFRcr-cys equations [figure]. In contrast, CKiD-U25 eGFRcys and eGFRcr-cys equations had a moderate underestimation of mGFR. P30 was moderate to high for all CKD-EPI, CKiD-U25 and EKFC equations. P30 for eGFRcr-cys equations was higher than eGFRcr and eGFRcys for all equations. Conclusion: Differences in performance between CKiD U25 vs. CKD-EPI and EKFC equations are observed for eGFRcys as well as eGFRcr. This suggests that population differences among the development datasets may vary by factors in addition to muscle mass. Additional research in young adults is needed to understand differences among populations and refine recommendations for use of eGFR equations. Funding: NIDDK Support