Daytime rhythm and treatment-related fluctuations of serum phosphorus concentration in dialysis patients

Serum phosphorus concentrations reflect a dynamic balance between generation, exchanges, and removal. Time-averaged phosphorus concentrations (TAC(phos)) reflect the overall exposure better than single time-point concentrations, especially in dialysis patients treated with an intermittent modality. The present study aimed to determine the daytime rhythm of phosphorus in dialysis patients and to compare the TAC(phos) in continuous ambulatory peritoneal dialysis (CAPD) and hemodialysis (HD) patients.

Serum concentrations of urea nitrogen, creatinine and phosphorus were determined at regular intervals in 10 healthy volunteers (HV), 8 CAPD patients and 10 HD patients. In HV and CAPD patients, blood was sampled at 08:30 (fasting), 09:30, 10:30, 12:30, 16:30, and 08:30 h the next day. In HD patients, blood was sampled before and after the midweek dialysis session and rebound was assessed at regular time-points post-dialysis. TAC(phos) were determined according to the trapezoidal rule.

Serum phosphorus levels show a daytime rhythm in CAPD patients, similar to what is observed in HV, with a nadir around 10:30 h and a rise in the afternoon. The magnitude of this daytime fluctuation is limited as compared to the treatment-related fluctuations in HD. These important fluctuations also translate in predialysis serum phosphorus concentrations overestimating the TAC. Remarkably, TAC(phos) were significantly lower in HD as compared to CAPD patients.

Daytime phosphorus rhythm is preserved in CAPD patients. These fluctuations are limited as compared to fluctuations in HD patients. Treatment-related fluctuation should be accounted for when comparing phosphorus exposure between different groups.