Length of interdialytic interval influences serum calcium and phosphorus concentrations.

BACKGROUND Higher levels of serum phosphorus and calcium are associated with increased haemodialysis (HD) patient mortality. Both of these factors act synergistically to promote vascular smooth muscle differentiation to an osteoblast-like phenotype and subsequent vascular calcification. The aim of this study was to investigate the influence of interdialytic interval on serum levels, as well as the influence of oral calcium-based phosphate binder load on the magnitude of the observed differences. METHODS We studied 100 patients undergoing HD three times per week, over a 2 week period. Haemoglobin, albumin, calcium and phosphate were measured pre-dialysis before each HD session. Oral phosphate binder usage was recorded. All patients were treated with dialysate containing 1.25 mEq/l calcium. RESULTS Both mean serum phosphate and calcium were higher after the long interdialytic interval (1.59+/-0.05 vs 1.45+/-0.04 mmol/l, P = 0.0005, and 2.46+/-0.03 vs 2.4+/-0.02 mmol/l, P = 0.001, for serum phosphate and uncorrected calcium, respectively). There were no significant differences in haemoglobin or serum albumin, indicating that variable dilution from an increased hydration status in the long interdialytic interval was unlikely to contribute to these observed differences. A total of 74 patients were treated with calcium-containing binders and 26 patients with sevelamer. Patients on sevelamer did not exhibit the observed cyclical increase in serum calcium seen in patients on calcium-containing binders (mean difference in serum calcium 0.09+/-0.01 mmol/l in the calcium-treated group vs 0.01+/-0.01 mmol/l in the sevelamer-treated patients, P = 0.0004). The increase in serum calcium after the long interval as compared with the short interval was proportional to the daily amount of the oral calcium-containing binder load ingested (r = 0.63, P<0.0001). CONCLUSION Cyclical differences in interdialytic interval and overall exposure to both dietary phosphate and oral calcium load influence serum levels. This may have consequences for registry reporting, therapy modulation and potentially the pathogenesis of accelerated vascular calcification seen in HD patients.