FGF21 and osteoporosis: Since serum FGF21 level is strongly dependent on RRF [20] that generally estimated by the 24-hour urine volume [43], we divided our osteoporosis HD patients into two subgroups based on the volume of 24-hour urine (24-hour urine volume ≤ 100 mL defined as no significant RRF [42]) and performed ROC analysis of FGF21 respectively to avoid relevant bias.