PROJECT SUMMARY Chronic kidney disease (CKD), or the gradual loss of kidney function, is the ninth leading cause of death in the US and poses an astronomical healthcare burden. CKD eventually leads to end-stage kidney disease (ESKD), requiring hemodialysis treatment (HD) or kidney transplant for survival. HD is the process of removing excess water, solutes, and toxins from the blood in patients, as kidneys can no longer perform these natural functions. In 2017 there were 746,557 Americans with ESKD, and 70% of these were on HD. Patients with ESKD accumulate fluid in between HD sessions, whose removal is a quintessential function of HD. Insufficient fluid removal on HD results in volume overload contributing to hypertension, heart failure and eventual cardiovascular mortality. Excess fluid removal results in hypotension, muscle cramps and compromises vitality after HD. Thus, it is critical to maintain dry weight, or weight without excess fluid in CKD patients. Despite the central importance of maintaining volume homeostasis in patients with ESKD, there are currently no quantitative standards for monitoring volume status of patients undergoing HD. The volume assessment of patients on HD remains the most challenging aspect of nephrology practice. This project aims to develop frequency-domain shortwave infrared spectroscopy (FD-SWIRS) for the unmet clinical need of HD volume status monitoring in patients with ESKD. The SWIR wavelength band (also called NIR II) is more sensitive to water and lipids compared to the visible or NIR bands, and can potentially provide deeper tissue measurements. FD-SWIRS will provide measurements of the absolute absorption and reduced scattering coefficients over a broad wavelength range (900-1310 nm). FD-SWIRS would represent a new technology, and to the best of our knowledge, no similar technologies have been used for HD monitoring. FD-SWIRS will provide three unique sources of label-free contrast for HD monitoring: 1) tissue molar water concentration changes calculated from broadband (900-1310 nm) absorption coefficients (µa), 2) broadband reduced scattering (µs′), and 3) bound water index (BWI), which utilizes subtle spectral shifts related to the binding of water molecules to proteins in tissue. We hypothesize that that these sources of contrast will accurately reveal volume status and provide a non-invasive indicator of extracellular versus intracellular water, which has been shown to be important for volume assessment. We will first develop an FD-SWIRS system and validate performance through rigorous system testing. A multi-layer inverse model will be developed to extract deep tissue optical properties. We will then perform pre, post and intradialytic measurements of FD-SWIRS in 50 dialysis sessions with the hypothesis that there will be alteration in FD-SWIRS signal between pre and post-HD time points. In addition, we posit that FD-SWIRS signal will change throughout dialysis session and may coincide or precede the...