The peculiarity of the proposed model is the possibility it offers to perform a real-time simulation enabling quantitative appraisal of hematochemical quantities whose direct measurement is prohibitive. These will be beneficial to dialysis therapy planning, reducing intradialysis complications and improving patients' quality of life.

Introduction: A reliable method of intradialysis calcium mass balance quantification is far from been established. We herein investigated the use of a single-pool variable-volume Calcium kinetic model to assess calcium mass balance in chronic and stable dialysis patients.Methods: Thirty-four patients on thrice-weekly HD were studied during 240 dialysis sessions. All patients were dialyzed with a nominal total calcium concentration of 1.50 mmol/L. The main assumption of the model is that the calcium distribution volume is equal to the extracellular volume during dialysis. This hypothesis is assumed valid if measured and predicted end dialysis plasma water ionized calcium concentrations are equal. A difference between predicted and measured end-dialysis ionized Correspondence to: S. Di Filippo, Division of Nephrology, A. Manzoni Hospital, Via dell'Eremo 9/11, 23900 Lecco, Italy. Email: difilippo.salvatore@libero.it Conflict of Interest: The Authors have nothing to disclose with regards of the present manuscript. Disclosure of grants or other funding: None List of abbreviations: 0-t, Initial and final values; Ca21d, Ionized dialysate calcium concentrations (mmol/L); Ca21di, Dialysate calcium concentration (mmol/L) at the inlet port of dialyzer; Ca21MB, Intra-dialysis ionized calcium mass balance (mmol); Ca21pw, Ionized plasma water calcium concentrations (mmol/L); Ca21pwi, Plasma water calcium concentration (mmol/L) at the inlet port of dialyzer; Ca21pwtM, End-dialysis measured plasma water calcium concentration (mmol/L) normalized at pH 7.40; Ca21pwtP, End-dialysis plasma water calcium concentration (mmol/L) predicted by the model; Ca21pwt(P-M), Difference between end-dialysis plasma water calcium concentration predicted and measured (mmol/L); Ca21uf, Outlet ultrafiltrate stream calcium concentration (mmol/L) normalized at pH 7.40; CaMB, Intra-dialysis global calcium mass balance (mmol); D 0 Ca, Calcium dialysance (L/min) corrected for cardiopulmonary and access recirculation; D 0 cond, Conductivity dialysance (L/ min) corrected for cardiopulmonar and access recirculation; KoAcond, "Intrinsic" dialysance for conductivity (mL/min); MCa21, Mobilization of calcium from extracellular fluid compartment (mmol); Na1d, Ionized dialysate sodium concentrations (mmol/L); Na1pw, Ionized plasma water sodium concentrations (mmol/L); Qbi, Inlet blood flow (mL/min); Qei, Inlet blood water flow (mL/min); Qdi, Inlet dialysate flow (mL/min); Cdi, Inlet dialysate conductivity (mS/cm); Qf, Total ultrafiltrate (L); QfECW, Ultrafiltrate removed from the extracellular fluid compartment (L/min); Qpwi, Inlet plasma water flow (L/min); SPVVCaKM, Single-compartment variable-volume calcium kinetic model; SPVV-NaKM, Single-compartment variable-volume sodium kinetic model; Td, Dialysis session duration (min); VCat, End-dialysis calcium distribution volume (L); Vut, End dialysis urea distribution volume (L); spKt/V, Single Pool Kt/V; a, Donnan factor Predialysis ionized plasma water concentration and total ultrafiltrate were the most imp...

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