During the study the investigators will collect multiple samples, each containing a value of SFI from the test device, and values of TFC and Z from the reference device. The investigators will enroll approximately 20-50 subjects, with the goal of at least 20 subjects completing the study. The statistical analysis will include data from all subjects that complete the study. All subjects will undergo dialysis in a clinic. Approximately half to a third will have both ESRD and Heart Failure (HF). The investigators will measure a sample from each subject every 15 minutes during a standard dialysis session, which typically lasts about 3 to 4 hours. Levels of extracted fluids, determined by the dialysis machine, will be recorded at the time each sample is measured.