Ultradian variation in serum phosphate concentration in patients on haemodialysis

To study how much the serum phosphate concentration could varyon the day before scheduled dialysis we obtained 24–37timed measurements during 24 h in nine patients on maintenancehaemodialysis. During the observation there was an increasein serum phosphate concentration of 0.239 ± 0.022, P/<0.01).Surprisingly six of the nine patients exhibited nine statisticallyand clinically significant nadirs with decrements of 0.21±0.04mmol/1 against an intra-assay standard deviation of about 0.05mmol/1. Five of the nadirs occurred at 11.40–14.50, mean12.59, and three at 19.00–21.30, mean 20.00. We foundno coinciding changes in the serum calcium concentration. Thesefindings indicate that serum phosphate concentrations do notalways reflect phosphorus balance or intake.