Original Article Residual renal function is an independent determinant of serum FGF-23 levels in dialysis patients

Background. Both poor residual renal function (RRF) and high fibroblast growth factor 23 (FGF-23) levels are associ-ated with arterial stiffness, left ventricular hypertrophy and increased (cardiovascular) mortality. Whether FGF-23 and RRF are interrelated is unknown. Methods. We performed a prospective observational co-hort study in 35 peritoneal dialysis (PD) patients with eval-uation at 1, 6, 12 and 24 months after start of PD. In addition, the role of RRF was assessed in a cross-sectional observational cohort study including 68 prevalent haemo-dialysis patients. Results. RRF significantly declined over time in PD pa-tients. This decline was parallelled by a significant increase of both serum phosphorus and FGF-23 levels. In the prev-alent dialysis cohort, RRF was found to be inversely asso-ciated with serum FGF-23 levels, independent of dialysis vintage, dialytic creatinine clearance, estimates of dietary phosphate intake (i.e. normalized protein nitrogen appear-ance), active vitamin D therapy and serum phosphorus and calcium levels. RRF, serum phosphorus and calcium levels and active vitamin D therapy explain 69 % of the variation in FGF-23. The 38 anuric patients had higher FGF-23 lev-els but similar serum phosphorus levels. Conclusions. We demonstrate an important association be-tween RRF and FGF-23, independent of classical determi-nants. This favours the hypothesis that the ailing kidney directly contributes to the raised FGF-23 levels. Whether FGF-23 is associated with poor outcomes independent of RRF, or vice versa, remains to be clarified