Project Summary/Abstract Late diagnosis of HIV is a significant public health problem due to increased risk for morbidity and mortality and the potential for HIV transmission. This Kirschstein NRSA Predoctoral Fellowship (PA-21-051) proposal aims to conduct an interdisciplinary multiphase explanatory sequential mixed-methods study to better understand the social-ecological factors associated with late HIV diagnosis, examine longitudinal HIV health outcomes (e.g., CD4 count and viremia), identify facilitators/barriers to HIV testing and healthcare engagement, and understand the impact of alcohol. This proposal responds to the National HIV/AIDS Strategy to identify missed opportunities for diagnosis, the Ending the HIV Epidemic Initiative to reduce late diagnosis, and the National Institute on Alcohol Abuse and Alcoholism (NIAAA)’s Strategic Plan to understand how alcohol affects HIV disease progression. Recent literature investigating factors associated with late is limited in United States populations. Moreover, there is not a consensus on the clinical definition of late diagnosis; common definitions are “traditional” CD4 count<200 cells/mm3 and “expanded” CD4 from 200 cells/mm3 up to 350 cells/mm3, though no studies to date have compared these definitions by significant covariates or longitudinal outcomes. The effect of alcohol is also understudied despite known associations with worse health and care engagement. To achieve the goals of this study, we will link two statewide datasets of people with HIV (PWH) in Florida: Medical Monitoring Project (MMP) and Enhanced HIV/AIDS Reporting System (eHARS) data collected between 2015-and 2019 (N=2,000). First, we will determine the incidence and describe demographic, behavioral, and psychosocial disparities between those with and without multiple definitions (traditional vs expanded) of late HIV diagnosis. Second, we will create generalized linear mixed effect models to compare longitudinal outcomes by late diagnosis and assess potential effect modification of heavy alcohol; we will use a subset of this data including only those diagnosed between 2015-and 2019 (N=200). Finally, we will explore reasons for late diagnosis, identify missed opportunities for testing, and determine new intervention points through qualitative interviews with 20-25 PWH with late HIV diagnosis in the Southern HIV and Alcohol Research Consortium (SHARC) contact registry (N=750). Findings will inform intervention and policy targets to reduce late diagnosis by identifying health disparities and missed opportunities for testing and healthcare engagement. A benefit of this award is training to establish the foundation of a successful career in epidemiologic research and intervention science such as 1) learning strengths, limitations, and implications of using multidimensional public health data to measure substance use and health outcomes; (2) understand the rationale for using different statistical methods to answer longitudinal re...