The study investigates the performance of a new dialyzer (Theranova 400) containing a membrane with increased pore sizes. The performance will be determined by the removal of middle molecules (with different molecular size) from the blood compartment. Three different Theranova 400 prototypes (AA, BB and CC) operated in hemodialysis mode will be compared with a Cordiax FX-80 dialyzer, operated in hemodialysis mode. Safety events and albumin loss into the dialysate will be monitored

Inclusion Criteria:

1. Patient has end-stage renal disease
2. Patient is 18 years of age or older
3. Patient is male or female
4. Patient, if female, is non-pregnant; and if capable of becoming pregnant, will be using a medically acceptable means of contraception during participation in the study (Note: Female capable of becoming pregnant [defined as a woman less than 55 years old who has not had partial or full hysterectomy or oophorectomy] must have a negative serum beta human chorionic gonadotropin [β-hCG] test within 2 weeks of first study treatment)
5. Patient has been receiving HD or HDF therapy (HDF patients are allowed if their treatments during the study can be safely and effectively performed with HD) for ≥3 months prior to study enrollment and is expected to survive for at least 12 months after enrollment
6. Patient has a stable functioning native fistula, Gore-Tex graft, or double-lumen central venous catheter capable of providing a blood flow rate of ≥280 mL/min (with an acceptable recirculation rate, such that solute removal is not likely to be affected) based on the judgment of the treating physician
7. Patient is in clinically stable condition as judged by the treating physician and as demonstrated by stable medical history for 30 days prior to enrollment, physical examination, and laboratory testing
8. Patient is willing to comply with the study requirements for therapy during the entire study treatment period
9. Patient is capable of providing written informed consent to participate in the study

Exclusion Criteria:

1. Patient is undergoing single-needle dialysis
2. Patient has an abnormal κ/λ ratio (less than 0.37, or greater than 3.1)
3. Patient has a known active infection and is currently receiving antibiotic treatment
4. Patient has known active cancer
5. Patient has a known positive serology test for human immunodeficiency virus (HIV) or hepatitis B, C or E
6. Patient has a known serious hemostasis disorder
7. Patient has a known monoclonal gammopathy (eg, monoclonal gammopathy of uncertain significance, smouldering [asymptomatic] multiple myeloma, symptomatic multiple myeloma, nonsecretory multiple myeloma, plasmacytomas, or plasma cell leukemia)
8. Patient has a known polyclonal gammopathy (eg, connective tissue disease, liver disease, chronic infection, lymphoproliferative disorder, or other hematologic condition)
9. Patient has any other known comorbidity that could, in the opinion of the Investigator, potentially conflict with the study purpose or procedures (eg, severe hypoalbuminemia or anemia)
10. Patient has a known significant psychiatric disorder or mental disability that could interfere with the patient's ability to provide informed consent and/or comply with protocol procedures
11. Patient has a history of non-compliance with the dialysis prescription, as assessed by the Investigator
12. Patient has participated in another interventional clinical study in the past 3 months, or is currently participating in another interventional clinical study