FGF23 and chronic kidney disease: Univariate analysis showed that there is a significant positive correlation between serum FGF23 levels and FEPi in CKD patients at stages 1 (r = 0.611, P<0.0001), 2 (r = 0.711, P<0.0001) and 3 (r = 0.613, P<0.0001), but not at stages 4–5 (r = 0.319, P = 0.0504) (Figure 3A–D).