PROJECT SUMMARY Although nearly 1.2 million people in the United States are indicated for HIV PrEP (pre-exposure prophylaxis), many fail to initiate or be retained in PrEP care. Prescription requests for PrEP prescriptions are on the rise, but it is yet unknown how many requested and approved prescriptions are actually received and then taken home by patients. When eligible patients present at pharmacies to pick up PrEP, they may opt not to purchase their approved prescription. This event is known as a prescription reversal. Prescription abandonment occurs when a patient never returns to purchase the prescription. Our previously published qualitative and quantitative research suggests that cost may be a driving factor in PrEP reversals and abandonments. In spite of growing recognition that out-of-pocket copays and deductibles may present barriers to PrEP initiation and retention in PrEP care, there are no published population-based estimates of PrEP prescription reversals or abandonments, which precludes our ability to understand how patients fall out of PrEP care. Thus, the aims of this study are to (1) determine the HIV PrEP prescription reversal, delay, and abandonment rates from 2015- 2017; and detect disparities in PrEP reversals and abandonment by (2) estimating the difference in HIV PrEP prescription reversal or abandonment by racial group, geographic region, sex, and out-of-pocket cost. For this first ever study of PrEP prescription reversals and abandonments, we will use a pharmaceutical claims database with the nation’s largest share of PrEP prescription claims matched to medical claims and individual patient demographics. Estimating prescription reversals and abandonment nationally is a novel way to identify who is lost to PrEP initiation or retention at the pharmacy point-of-sale, and has implications for expansion of national safety-net (e.g., Ryan White) and medication assistance programs. Having valid estimates of prescription reversals can help us refine our estimates of who is truly retained in care, and at what endpoints patients fail to progress in the PrEP care continuum. This analysis, developed in response to patient feedback, is significant in light of major media coverage and patient protests of exorbitant PrEP costs at the point-of-sale, and the paucity of research on this real-world issue. The results will form preliminary data so that we can determine where geographically PrEP reversals happen, and among whom, and propose an R34 or R01 to qualitatively and quantitatively investigate why reversals are happening and how to intervene. Our findings will inform subsequent R series interventions by identifying cost thresholds for abandonment, and refine which specific subgroups are at highest risk of not being retained in PrEP care. The long-term goal of this research is to improve PrEP retention and ultimately decrease HIV incidence by reducing barriers to PrEP among high-risk populations. This study aligns with the high pri...