FGF21 and osteoporosis: In the two subgroups based on 24-hour urine volume in HD patients, the area under the curve (AUC) of FGF21 in predicting osteoporosis were 0.657 (95 % CI, 0.558 to 0.755, P = 0.001) and 0.691 (95 % CI, 0.606 to 0.775, P = 0.001), respectively (Table 6).