Calcium balance is an overall assessment of the net
calcium taken in minus the net
calcium taken out. It can only be assessed when patients are in steady state and requires complicated
isotope methods that can simultaneously assess intestinal absorption and endogenous secretion, urinary and stool excretion, bone
calcium uptake and removal, and
dialysate calcium removal. By virtue of the need for steady state, formal balance studies cannot be accurately carried out in patients on dialysis. However, many of the components of
calcium balance have been assessed. Importantly, because 99% of
calcium is in bone, studies must accurately assess both the rapidly exchangeable
calcium from the bone surface and the net bone
calcium balance that results from the difference in bone formation minus resorption. While it is tempting to adjust the
dialysate calcium concentration to correct the net positive
calcium balance that is likely present in patients who receive
calcium-based
phosphate binders, the reality is that the highly variable, yet important, role of bone cannot be easily assessed at the bedside. Thus, it is best to prevent the
calcium overload in the first place by avoiding high-dose
calcium-based
phosphate binders and optimizing bone remodeling.