Serum phosphorus adds to value of serum parathyroid hormone for assessment of bone turnover in renal osteodystrophy

It is well-established that parathyroid hormone (PTH) correlates with the level of bone turnover in patients with chronic kidney disease stage 5D (CKD-5D). Hyperphosphatemia is a well-established complication of end-stage renal disease and is usually attributed to dietary intake. This study evaluates the relationship between serum phosphorus levels and bone turnover in patients with CKD-5D. 93 patients with CKD-5D from the Kentucky Bone Registry who had sequentially undergone anterior iliac bone biopsies were reviewed. Undecalcified bone sections were qualitatively assessed for turnover and placed into a group with low turnover and a group with non-low (normal/high) turnover. Results of PTH and phosphorus concentrations in blood drawn at the time of biopsies were compared between the groups. PTH and phosphorus levels were significantly higher in the non-low turnover group compared to the low turnover group. Cutoff levels for PTH and phosphorus were tested for predictive power of bone turnover. Both PTH and phosphorus correlated with turnover. Adding serum phosphorus to serum PTH enhanced predictive power of PTH for low turnover. The vast majority of patients with serum phosphorus levels ≥ 6.0 mg/dL had non-low turnover, while the majority of those with low turnover had phosphorus values < 6.0 mg/dL. Classification and regression-tree analysis showed that elevated serum phosphorus (> 6.2 mg/dL) in patients with PTH < 440 pg/mL was helpful in diagnosing nonlow turnover in this range of PTH. In patients with PTH ranges of 440 - 814 pg/mL, serum phosphorus levels > 4.55 mg/dL ruled out low turnover bone disease. This suggests that not only dietary intake but also bone affects serum phosphorus levels.