It is thought that C-terminal FGF-23 better reflects the biologically functional FGF-23 molecule that reduces the reabsorption of P in kidney tubules.27 In their study on 81patients with CKD younger than 25 years, Yasin et al. reported that FGF-23 levels varied depending on age.28 The type of FGF-23 kits used, the ages of the patients included, and their growth status may have yielded different results. This evidence concerns the gene FGF23 and chronic kidney disease.