Introduction: Prognosis of chronic kidney disease (CKD) is closely related to early diagnosis and initiation of nephroprotective measures. Fibroblast Growth Factor 23 (FGF23) is well described as an early marker that increases proportionally to worsening CKD stages in adults. The aim of this study is to investigate the association between FGF23 and glomerular filtration rate (GFR) in children.
Methods: Children aged 5 to 20 years old requiring kidney function assessment following solid organ transplantation or primary nephropathy were recruited prospectively in a tertiary hospital of Switzerland for a gold-standard measurement of their GFR, with simultaneous measurement of plasma FGF23. 
Results (preliminary): 159 clearances in 109 children post solid organ transplantation (43%), with primary nephropathy (39%) or another cause for chronic kidney disease were analyzed. 42% had stage I CKD, 43% had stage II CKD and 15% had stage III or IV CKD. FGF23 was significantly higher in stage III or IV CKD (mean: 282.02 UI/ml ± 174.71) compared to stage I (mean: 136.82 UI/ml ±157.51; p<0.001) or stage II (mean: 106.56 UI/ml ±66.40; p<0.001). 

There was a moderate inverse association between FGF23 and GFR (r=-0.364, 95% confidence interval (CI): -0.520 to -0.186). FGF23 correlated with the urine protein-to-creatinine ratio (r=0.451, CI: 0.242-0.621) and parathyroid hormone levels (r=0.430, CI: 0.220-0.602). The area under the ROC curve for FGF23 to discriminate CKD stages I-II versus III-IV was 0.864 (95% CI: 0.776-0.953). 

Conclusion: FGF23 is a good predictor of CKD stages in children with solid organ transplantation or primary nephropathy and is significantly associated with decreased GFR. Further studies are needed to evaluate FGF23 as a marker of CKD progression.