Serum level of fibroblast growth factor 23 in maintenance renal transplant patients

The discovery of fibroblast growth factor 23 (FGF23) provides a new conceptual framework that improves our understanding of the pathogenesis of post-transplant bone disease. Excess FGF23 is produced in the early post-transplant period; levels return to normal in the months following transplant. However, few manuscripts discuss FGF23 levels in stable long-term renal transplant recipients. We performed a cross-sectional observational study of 279 maintenance kidney recipients with chronic kidney disease (CKD) Stages 14 and stable allograft function who had received their transplant at least 12 months previously. We calculated the estimated GFR (eGFR) using the MDRD4 equation. FGF23, parathyroid hormone (PTH) and phosphorus values were higher in more advanced stages, while the serum calcitriol levels and the phosphate reabsorption rate were lower. A significant inverse correlation was found between eGFR and FGF23 (r 0.487; P 0.001), PTH (r 0.444; P 0.001), serum phosphate levels (r 0.315; P 0.001) and fractional excretion of magnesium (r 0.503; P 0.001). Multivariable analysis showed that increased time on corticosteroids (P 0.001), PTH (P 0.001), serum phosphate (P 0.003), decreased serum calcitriol (P 0.049) and estimated glomerular filtration (P 0.003) rate were associated with high FGF23 levels. In contrast with pre-transplant patients and first year post-transplant patients, higher FGF23 values were not correlated with increased phosphate excretion. An elevated phosphate reabsorption rate was associated with decreased PTH (P 0.001) and calciuria (P 0.028) and increased serum calcitriol (P 0.009), plasma bicarbonate (P 0.024) and estimated glomerular filtration (P 0.003). Serum FGF23 concentrations remain increased in long-term kidney graft recipients, even in the early stages of CKD. It remains to be seen whether measures aimed at reducing serum levels of PTH and phosphate and/or corticosteroid doses might help to lower serum FGF23 and whether this will improve kidney recipient outcomes.