FGF23 and chronic kidney disease: C-terminal FGF23 fragments as a proportion of the total FGF23, determined by volumetric quantitation of Western Blot analysis at appropriate molecular weights of 14 kDa (cFGF23) and 32 kDa (iFGF23), decline as renal function falls: 21–56% with normal kidney function, 19% in pre-dialysis CKD patients, and 5% in HD patients [21].