FGF23 and anemia (phenotype): Compared to untreated subjects, those receiving ESAs had a lower median (IQR) eGFR (29 [20, 39] vs. 50 [38, 63] ml/min/1.73 m2, p < 0.001), a higher prevalence of anemia (51% vs. 17%, p < 0.001), a higher median total FGF23 concentration (254 [129, 459] vs. 113 [79, 184] RU/ml, p < 0.001), and a higher median total FGF23 SDS for age (4.3 [1.9, 7.2] vs. 1.4 [0.4, 3.4], p < 0.001) (Table 3).