ABSTRACT Adequate oxygen inside cells is critical for healthy organ function, and the main goal of clinicians in transfusion medicine, emergency medicine, and critical care is to restore and maintain cellular oxygen levels in patients. However, at this time, there is no clinical device that directly measures cellular oxygen levels. While RBC transfusions can be life-saving, they have inherent risks and high costs. Transfusion decisions are usually based on the degree of anemia, assessed by a hematocrit measurement, with the assumption that a red blood cell (RBC) transfusion will increase oxygen delivery to cells. Without a measure of cellular oxygenation, it is currently unknown if RBC transfusions improve the delivery of oxygen to tissues in general and in each individual who receives a transfusion. By measuring and analyzing full optical spectra in the visible and near-infrared regions at multiple source-detector separations, Opticyte?s CellSat? 100 will quantify myoglobin saturation, or cellular saturation (ScO2), in muscle cells separately from hemoglobin saturation in the vasculature. ScO2 is directly related to intracellular pO2 via the myoglobin oxygen dissociation curve. ScO2 monitoring has the potential to help optimize transfusion decisions by identifying anemic patients with inadequate oxygen delivery, as evidenced by low ScO2. In our Phase II SBIR project, we have designed and built a proof-of-concept prototype for the CellSat? device and have obtained promising spectra from the hand. We have also successfully made ScO2 measurements from spectra acquired from the hand with our laboratory prototype. We have completed a human factors study on the disposable sensor and have interviewed key opinion leaders in the hospital setting to define the use environment and workflow for the device. Our Phase IIB activities will revolve around development of the CellSat? 100, through two prototype stages to pre-manufacturing. We will also perform an observation study in the outpatient and inpatient oncology services and in the ICU to demonstrate the clinical value of ScO2 measurement to optimize transfusion decisions. The Specific Aims of this proposal are to: 1) develop and test the Alpha Prototype that proves the device design; 2) develop and test the Beta Prototype that proves the product is ready; 3) integrate algorithm into Beta Prototype; 4) define the relationship between anemia and cellular oxygenation, and 5) transfer the CellSat? 100 to manufacturing. The CellSat? 100 will provide unprecedented information about cellular oxygenation that has major implications for both research and clinical care. The device has broad applicability to transfusion medicine, emergency medicine, critical care, and surgery, and will be useful in ambulances, emergency departments, intensive care units, and operating rooms.