This is a randomized controlled clinical trial that will analyze the impact of high volume online HDF in comparison to high-flux HD on measured physical activity.

HD-FIT is a randomized controlled clinical trial that will analyze the impact of high volume online HDF in comparison to high-flux HD on measured physical activity (number of steps measured in the dialysis day) as a primary endpoint. The analysis of additional accelerometer data, HRQOL and time to recover from a dialysis session will be used as other outcomes. The study will also capture safety data, based on intradialytic events, hospitalization and mortality. Biochemical (according to the local requirements) and drug prescription data will be monitored for pharma-economic analysis. Serum samples will be collected for future analysis of additional biomarkers.

In summary, patients will go through a 4 week run in period on high flux HD. After this, patients will be randomized to the intervention of high volume online HDF for 6 months, or will continue on high flux HD; both groups will be observed for a 12 month follow up period. Patients will be evaluated for physical activity, HRQOL, laboratories and other measures at the baseline, 3 month, and 6 month time points. Evaluation will be repeated in 3 and 6-months. HRQOL, laboratories, and other outcomes will be tracked over the post-interventional follow up period that lasts 12 months after randomization.

Data will be analyzed according to an intention-to-treat principle (i.e., according to assigned instead of received treatment). With an enrollment target of 110 participants in each arm (considering a drop out of 20%) we estimate the 86 patients will complete the follow up in each arm, what will provide the trial a 90% power to detect a 20% effect with respect to the primary outcome (predefined as a 20% increase in average total steps at the dialysis day in the HDF group compared to the HD group). Because of the nature of the intervention, it will not possible to blind the patients, the local study nurses, or the investigators for the treatment assignment.

Fourteen dialysis centers will be invited to participate. Each participating center will receive two 5008S machines and Cordiax dialyzers to be used in the study of 10 patients in each center. Additional patients can be randomized per center with approval of the steering committee. Centralized randomization will based on a 1:1 protocol, stratified by participating center.

Inclusion Criteria:

• Patients treated three times per week with HD for at least 3 months and up to 24 months;
• Clinically stable according to the evaluation of the investigator;
• Kt/V of > 1.2;
• Patients with an arteriovenous fistula/graft and permanent catheters with adequate flow.

Exclusion Criteria:

• Age below 18 years;
• Life expectancy less than 3 months because of nonrenal disease;
• Participation in another clinical intervention trial;
• Severe non-compliance regarding frequency and duration of dialysis treatment;
• Patients with severe limitation to mobility (amputated, neurologic and muscular disorders) will be excluded from the study.