Today, haemodialysis is a recognized standard treatment for patients with chronic kidney disease stage 5. During the haemodialysis treatment session, blood passes from the patient through the extracorporeal circuit and is then returned. The dialyzer represents the greatest surface are of the extracorporeal circuit, as dialysis treatment is essentially based on the removal of small molecular weight solutes down along a concentration gradient, and this depends upon surface area. The ELISIO-H dialyzer differs in design to our current standard dialyzer, the FX100, by having fibers of a greater internal diameter, which potentially allows more internal haemofiltration, leading to an improved clearance of larger molecular weight solutes. It is now thought that these so called "middle molecular weight" solutes are more important in contributing to the clinical condition termed azotaemia, rather than smaller solutes such as urea.

The investigators therefore wish to study the clearance of middle sized molecules between the different dialyzers.

Stable chronic kidney dialysis patients, currently dialyzing on the main Royal Free hospital dialysis unit will be asked to participate in the study. It is aimed to recruit 15 patients currently dialysing using the Fresenius FX100 dialyzer, who have used Fresenius polysulphone membranes for > 3 months.

During a mid week dialysis session, dialysis adequacy will be assessed by on line clearance, and samples of both blood and dialysate taken to assess, both clearances and bio-compatibility.

Thereafter patients would be switched to dialyse using the ELISIOTM-H dialyzer, but continue with the same dialysis prescription, and after 3 months, measurements repeated

Assessments Primary Clearances : on-line Kt/V, dual pool Kt/V, phosphate, b2microglobulin, cystatin C Secondary Serum :isoprostane generation complement activation platelet activation - coagulation activation - thrombophilic markers - Other Anticoagulation requirements & clotted circuits erythropoietin requirements, phosphate control therapy CRP

Inclusion Criteria:

• 15 adult patients currently dialysing using the Fresenius FX100 dialyzer, who have used Fresenius polysulphone membranes for > 3 months, who are stable and able to provide informed consent.

Exclusion Criteria:

• using other dialyzers, unable to provide informed consent, unstable on dialysis or have problematical vascular access