Notes: SUMMARY:  Renal replacement therapy is frequently required for critically ill patients with a high risk of bleeding. Conventional heparinization strategies to prevent extracorporeal blood circuit clotting can cause significant haemorrhage in such patients because of systemic anticoagulation. Regional citrate anticoagulation (RCA) is a well-established technique that minimizes this complication by the decalcification of blood in the extracorporeal circuit such that it is incapable of clotting. To date, there are no reports on the use of RCA for sustained low-efficiency dialysis/diafiltration (SLED), a hybrid therapy that involves the use of conventional haemodialysis machinery to deliver lower solute clearances over prolonged periods of time. In preparation for clinical study, an in vitro simulation of SLED was devised (blood substitute flow 250 mL/min, dialysate flow 200 mL/min, predilution haemofiltration 100 mL/min). Blood substitute was decalcified by an infusion of 4% trisodium citrate (TSC) proximally into the extracorporeal blood circuit, with partial restoration of calcium homeostasis from dialysate containing ionized [Ca2+] at 0.9 mmol/L. This simulation was used to establish first the 4% TSC requirement for therapeutic decalcification, and second the associated changes in ionized [Ca2+] and [Mg2+] within the blood substitute from chelation with citrate and subsequent removal of the resulting divalent cation-citrate complex. Serial measurements of blood substitute [Ca2+] from strategic points along the extracorporeal circuit showed therapeutic decalcification was not achieved with 4% TSC infusion rates up to 400 mL/h, and extrapolation of experimental results suggests that 450 mL/h will be required. Under these conditions, ionized [Ca2+] and [Mg2+] in the blood substitute venous return and would be 0.42 and 0.2 mmol/L, respectively, with 0.35 mmol of citrate being returned per minute via the blood substitute venous return. These results were modelled for various changes in SLED operating parameters, and discussed in detail. An appropriate regimen for 4% TSC infusion and divalent cation replacement is proposed for clinical study in the future.