FGF23 and chronic kidney disease: Figure 3(a)). Levels of FGF23 were significantly higher since CKD stage 2 (reference values = 36 ± 11, pg/mL; stage 2 = 46 ± 18, pg/mL, P < .001; stage 3 = 57 ± 28, pg/mL, P < .002; stage 4 = 136 ± 58, pg/mL, P < .001.  Figure 3(b)) with no further increment in later CKD stage. Multivariate analysis performed to identify the best predictors of s-Klotho (Table 4) and FGF23 (Table 5) included eGFR and serum calcium for s-Klotho and eGFR and 1,25D serum level for FGF23.