The choice of the dialysate calcium concentration in the management of patients on haemodialysis and haemodiafiltration.
The choice of an appropriate dialysate calcium (Ca) concentration is crucial in the management of dialysis patients: an excessive Ca load has been associated with vascular calcifications, whereas Ca depletion can worsen secondary hyperparathyroidism (HPT) and decrease bone mass. In haemodialysis (HD), Ca transfer by diffusion depends on the concentration gradient between dialysate and blood, and a gain of Ca is expected when the dialysate Ca is >1.5 mmol/l. However, Ca losses by convective transport (ultrafiltration rate) can even exceed the amount of Ca gained by diffusion. Dialysis Ca balances, in normocalcaemic patients with a mean weight loss of 2-4 kg per session, have been shown to be positive, moderately negative or clearly negative using a dialysate Ca of 1.75, 1.50 or 1.25 mmol/l, respectively. Serum ionized Ca increases during sessions with a dialysate Ca of 1.5 and 1.75mmol/l, and decreases to the lower limits of normal after HD with 1.25 mmol/l. In haemodiafiltration (HDF), Ca mass transfer is strongly affected by the Ca content in the replacement solutions. Bicarbonate-containing bags are Ca free so that dialysate Ca needs to be increased above 1.75 mmol/l to overcome the convective losses and avoid markedly negative balances. Ca mass transfer in HDF is also affected by the infusion mode. Ca balance in post-dilution HDF, for a given concentration gradient between blood and dialysate, does not differ from HD. Conversely, in pre-dilution HDF, dialysate Ca concentration should be increased by approximately 0.25 mmol/l to maintain comparable balances. A given dialysate Ca concentration should be prescribed considering the dialysis Ca mass balance, other concomitant therapies (Ca salts, vitamin D metabolites) and the type of bone disease. The current strategy of maintaining normal Ca levels by ensuring an adequate intestinal Ca absorption with large doses of Ca-containing phosphate binders and concomitantly avoiding positive Ca balances from dialysis by using a low Ca dialysate has been questioned recently because of the risk of either worsening HPT or accelerating the progression of vascular calcifications. A dialysate Ca of 1.5 mmol/l seems to be suitable for the majority of patients on HD or post-dilution on-line HDF because the moderately negative dialysis balances can be easily counterbalanced by the administration of mild doses of Ca-containing phosphate binders in order to ensure a neutral total body Ca balance. If necessary, aluminium-free and Ca-free binders can be added to achieve a satisfactory control of hyperphosphataemia, while avoiding an excessive Ca load.