Calcium mass balance (CaMB) is determined by the difference between Ca absorbed between dialyses (CaAbs) and the Ca removed during dialysis (JdCa2+). A mathematical model to quantify (1) CaAbs as a function of Ca intake (CaINT) and the doses of vitamin D analogues, and (2) JdCa2+ as a function of Ca2+ dialysance, the mean plasma Ca2+ (MCpiCa2+) minus dialysate Ca2+ (CdiCa2+), ultrafiltration rate (Qf) and treatment time is developed in this paper. The model revealed a basic design flaw in clinical studies of Ca-based as opposed to non-Ca-based binders in that CdiCa2+ must be reduced with the Ca-based binders in order to avoid a positive CaMB relative to the non-Ca-based binders. The model was also used to analyze CaMB in 320 Renal Research Institute hemodialysis patients and showed that all patients irrespective of type of binder were in positive CaMB between dialyses (mean ± SD 160 ± 67 mg/day) with current doses of vitamin D analogues prescribed. Calculation of the optimal CdiCa2+ for the 320 Renal Research Institute patients revealed that in virtually all instances, the CdiCa2+ required for neutral CaMB, where Ca removal during dialysis was equal to Ca accumulation between dialyses, was less than 2.50 mEq/l and averaged about 2.00 mEq/l. This sharply contradicts the recent KDIGO (Kidney Disease: Improving Global Outcomes) Clinical Practice Guideline for the Diagnosis, Evaluation, Prevention, and Treatment of Chronic Kidney Disease – Mineral and Bone Disorder, that suggests a CdiCa2+ of 2.5–3.0 mEq/l. Review of the KDIGO work group discussions shows that this discrepancy stems from the unwarranted work group assumption that intradialytic CaMB is zero. We strongly believe that this guideline for dialysate Ca2+ is inappropriate and should be modified to more realistically reflect the needs of dialysis patients.

Keywords:
Calcium mass balance, Dialysis, Calcium absorption, Vitamin D analogues, Calcitriol, Calcium flux
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© 2010 S. Karger AG, Basel
2010
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