Ultradian variation in serum phosphate concentration in patients on haemodialysis

To study how much the serum phosphate concentration could vary on the day before scheduled dialysis we obtained 24-37 timed measurements during 24 h in nine patients on maintenance haemodialysis. During the observation there was an increase in serum phosphate concentration of 0.239 +/- 0.022, P < 0.01). Surprisingly six of the nine patients exhibited nine statistically and clinically significant nadirs with decrements of 0.21 +/- 0.04 mmol/l against an intra-assay standard deviation of about 0.05 mmol/l. Five of the nadirs occurred at 11.40-14.50, mean 12.59, and three at 19.00-21.30, mean 20.00. We found no coinciding changes in the serum calcium concentration. These findings indicate that serum phosphate concentrations do not always reflect phosphorus balance or intake.