Variability in calcium, phosphorus, and parathyroid hormone in patients on hemodialysis.

Calcium, phosphorus, and parathyroid hormone (PTH) levels are routinely measured in patients undergoing chronic hemodialysis. Medications, diet, and dialytic therapies are modified based upon these lab values to achieve specific goal values in the hope of improving outcomes. However, the variability of these values in patients undergoing chronic hemodialysis has only been rarely studied. We prospectively investigated the variability of these measures in 35 patients undergoing chronic hemodialysis as well as the impact of this variability on clinical decision-making in a prospective manner over a month. There is significant session-to-session variability in phosphorus and PTH values (mean coefficient of variations [CV] of 0.19 and 0.31, respectively). Calcium variability is much lower (mean CV of 0.05). Not surprisingly, the CV for all values is increased during the long interdialytic interval. The impact of this variability on clinical decision-making was analyzed. The variability in calcium, phosphorus, and PTH values would lead to a different clinical decision in 23.6%, 41.2%, and 39.7% of different session lab values. We also investigated the variability of these lab measures over a year in these patients and found that the session-to-session variability was very similar to the month-to-month variability. The high degree of variability of these parameters has important implications for clinical decision-making and for implementation of pay-for-performance measures.