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Background. There is no consensus regarding the optimal dialysate calcium concentration (DCa) during haemodialysis (HD). Low DCa may predispose to acute arrhythmias, whereas high DCa increases the long-term risk of soft tissue calcifications.

Methods. Twenty-two HD patients treated in four dialysis centres underwent two HD sessions, respectively, with 1.5 and 1.25 mmol/L total DCa. Calcium mass balance (CMB) was calculated from ionized calcium (iCa) in the dialysate and blood at the start and end of each run, using a kinetic

formula to define the mean concentrations in the blood and dialysate and then estimating CMBs over the entire treatments.

Results. Mean blood iCa levels increased using 1.5 DCa, whereas they remained unchanged using 1.25 DCa. Diffusive CMB positively correlated with the dialysate/blood iCa gradient. With 1.5 DCa, diffusive CMBs were strongly positive at the blood side and negative at the dialysate side, indicating transfer from dialysate to blood. With 1.25 DCa,

despite a negative dialysate/blood iCa gradient, diffusive CMB was slightly positive in blood and negative in dialysate. The global balances based on both the convective and diffusive components showed a positive net transfer of Ca from dialysate to blood with 1.5 DCa and an approximately neutral Ca flux with 1.25 DCa. Conclusions. While CMB is nearly neutral when using 1.25 DCa, the use of 1.5 DCa results in a gain of Ca during HD. The risks associated with Ca load should be considered in the choice of DCa prescription for HD but need also be weighed against the risk of worse haemodynamic dialysis tolerance.