 Apheresis blood donation maximizes an individuals donation by selecting specific blood components that are used to save patient lives. Dramatic increases in the number of apheresis procedures performed each year are apparent both domestically and internationally. Apheresis requires the use of citrate anticoagulation, a substance that confers its anticoagulant effect through chelation of cations, like calcium. A small number of cross-sectional studies have reported that intermittent exposure to citrate through apheresis is associated with significant declines in donor bone mineral density BMD. In contrast, oral potassium citrate of much lower dose has been used to treat low bone density with well-documented efficacy. The impact of citrate exposure during apheresis, either positive or negative, is important given that BMD is a significant risk factor for low trauma fracture, a problem that affects more than 2M people in the U.S. annually. It is ultimately unknown what effect repeated apheresis has on skeletal health. To address this knowledge gap, we will perform a Phase IV clinical trial of apheresis blood donors using a cross-disciplinary, multi-institutional team. As apheresis blood donation continues to affect more people, the importance of understanding the effects of repeated exposure to citrate on donor skeletal health is essential in protecting this precious community resource. 