Purpose: Pediatric glomerular filtration rate estimation (eGFR) formulas were developed in cohorts of chronic kidney disease (CKD) patients excluding transplant patients. Previous studies compared the performance of various creatinine-based equations in pediatric kidney transplant (kTx) recipients. Although cystatin C is known to improve eGFR in CKD patients, the performances of cystatin-based equations and composite (creatinine and cystatin C based) equations have not been systematically evaluated. The aim of this study is to assess the performance of creatinine-based equations, cystatin-based equations and composite equations to estimate GFR in pediatric kTx recipients.
Methods: We included pediatric kTx recipients (<18 years old) who underwent plasma iohexol clearances with concomitant serum creatinine and cystatin C assessment between January 2017 and December 2022. Performances of eight eGFR formulas were assessed by calculating the precision (median bias, and square of the linear correlation coefficient), and the accuracy (P30, P10, P7,5, root mean square error [RMSE]) :  three creatinine-based equations (Schwartz 2009, EKFC, U25 creat), two cystatin-based equations (Cystatin 2012, U25 cystatin), and three composite (creatinine and cystatin-based) equations (BUN-creat-cyst, FAS, U25 mean).
Results: 248 iohexol clearances were performed in 109 children, 64,2 % were male and the median age at measurement was 15.2 years (2.76, 18.0). The median mGFR was 64.5 ml/min/1.73m2  (10.0, 136). Composite equations had a better performance than creatinin-based or cystatin-based equations. U25 mean, BUN-creat-cyst, and FAS had a median bias of -6,90, -6,81 and 7,26 ml/min/1.73m2 and a P30 of 0.98, 0.96 and 0.98. Among creatinin-based formulas, we confirmed that the Schwartz 2009 formula performed the best with a median bias of 8.97 mL/min/1.73m2 and a P30 of 94%, similar to what was reported in CKD cohorts.
Conclusion: Composite formulas presented higher performances to estimate GFR in pediatric kTx recipients. Among creatinine-based equations, Schwartz 2009 had an acceptable performance. Measuring GFR via exogenous markers remains beneficial in this population.