Thus, using a polyclonal enzyme-linked immunosorbent assay (ELISA), which recognizes multiple epitopes on the VDBP and thus unaffected by VDBP isoforms, for quantifying VDBP [41], an ELISA for directly measuring free-25(OH), and using estimated free-25(OH)D as a comparison, the aim of our study was to determine total-25(OH)D, directly measured free-25(OH)D, calculated free-25(OH)D, VDBP, and their relationship with biomarkers of mineral metabolism in children with CKD stage 2–3, on dialysis, and with functioning renal transplant. Here, GC is linked to chronic kidney disease.