Calcium Exposure and Removal in Chronic Hemodialysis Patients

The risks associated with calcium exposure in chronic hemodialysis (HD) patients are becoming increasingly apparent. Current K/DOQI guidelines recommend an absolute maximum elemental calcium load of 2,000 mg/d, including calcium-containing medication and a maximum dialysate calcium concentration of 1.25 mmol/L (to avoid intradialytic calcium loading). The goal of this study was to characterize the total exposure to calcium from all sources that chronic HD patients are exposed to. We studied 52 patients. Each was requested to complete a 3-day food diary for analysis of daily calcium intake; 24-hour urine collections were taken and analyzed for calcium content. All patients underwent HD using Hospal Integra (Lyon, France) dialysis monitors, bicarbonate buffering, and dialysate sodium and calcium concentrations of 134 mmol/L and 1.25 mmol/L, respectively. Blood was sampled before and after HD for total serum calcium, albumin, bicarbonate, and phosphate, in addition to ionized calcium level measured at the bedside using a portable electrolyte analyzer. Calcium flux was determined from measurements of ionized calcium levels in dialyzer inlet samples and those in continuous partial waste dialysis collection (with reference to total waste dialysate and ultrafiltration volumes). There was marked interpatient variability of total calcium exposure; the mean was 2,346 ± 293 mg (range, 230 to 7,309 mg) per day. The majority of enteral calcium exposure was from calcium-containing phosphate binders, with diet providing only a mean load of 581 ± 34 mg (range, 230 to 1,309 mg). Calcium removal was evident in 83% of patients. Mean calcium flux was −187 ± 232 mg (range, −486 to 784 mg). There was a linear correlation observed between the amount of calcium removed during dialysis and the predialysis ionized plasma calcium concentration, r² = 0.42, P < .001 (calculated from actual measured dialysate ionized calcium concentration). This shows that calcium flux across the dialysis membrane is determined by the diffusion gradient. The amount of calcium removed during dialysis was found to be independent of exogenous calcium load. These results support previous reported data showing that the majority of HD patients are continually experiencing calcium overload. This may have a contributory role in the development of vascular calcification. In contrast to recent K/DOQI recommendations, an upper dialysate concentration of 1.25 mmol/L may not be ideal for every patient. To minimize the effects of exogenous calcium overload, dialysate concentrations should be prescribed with reference to plasma calcium levels.