Phosphorus dynamics during hemodialysis.

We studied phosphorus (P) dynamics and its relation to urea dynamics in a wide range of dialyses by measuring predialysis and postdialysis serum P levels and all removed P and urea in dialysate during 455 hemodialyses. Dialyses were performed at different frequencies (range 3-6 treatments/wk); duration of dialysis (t) (range 80-560 minutes), varied blood and dialysate flow, and with high-flux and low-flux membranes. Kt/V-P, Kt/V-urea, weekly removal of P-and urea and removal volumes (Vr) and their relationships to varying dialyses, and predialysis concentrations, and protein catabolic rates were studied in linear and multiple regression analyses. A weekly dialysis time of > 30 hours was needed to maintain serum P concentration normal without the use of phosphate binders. Vr-P as a percentage of body weight was dependent on predialysis serum P and increased steeply as predialysis serum P decreased and dialysis time was prolonged. There was no relationship between Vr-urea and Vr-P. Phosphorus removal per week was mainly dependent on weekly frequency, and time on dialysis and > 38 h/wk were necessary to remove the recommended P intake. Phosphorus shows highly variable dynamics during dialysis. The body maintains extracellular P concentration by releasing P from large compartments when the dialysis time is prolonged and the serum concentration of P decreases during dialysis. Vr-P shows huge variation between patients and in an individual patient, depending on predialysis serum P. Kt/V is inaccurate in describing P removal. To remove P efficiently, it is most important to perform long and more frequent hemodialysis.