PTH and endothelial dysfunction: This was not applied to stage 4–5 CKD patients where serum phosphorus (r = − 0.53, P 0.001), intact PTH (r = − 0.53, P 0.001), uric acid (r = − 0.5, P 0.001), and measured GFR (r = 0.59, P 0.001) were the highest correlates to FMD; the Nephron index had the weakest correlation (r = 0.28, P = 0.02) and is not predictive of endothelial dysfunction.