Project Summary Antiretroviral therapy (ART) adherence among nursing home (NH) residents with HIV is less than optimal; in the literature 21% of people with HIV had no ART from their NH, which is in contrast to the 87.5% adherence among community dwelling older people with HIV. ART maintains viral suppression, prevents worsening of chronic conditions and early mortality, and stopping ART is rarely indicated. Therefore optimal ART adherence in the NH setting can only be achieved by understanding facilitators and barriers both documented in claims and perceived by older people with HIV. This mixed methods study will assess ART adherence during the community-to-long-stay-NH-transition period in three aims: Aim 1 will identify person-level, and NH-level, factors associated with changes in ART adherence in socioeconomically diverse Medicare and Medicaid special needs plan (SNP) claims. Aim 2 will assess perceptions and experiences of long-stay NH residents with HIV through qualitative interviews and Aim 3 will examine concordant and discordant factors associated with ART adherence by triangulation of Aims 1 and 2. Quantitative analyses will use two databases 1) Medicare 5% enhanced sample; 2) claims from a New York City-based Medicaid HIV SNP. Logistic and linear regression with generalized estimating equations will examine the change in ART adherence from the 6 months prior, to the first three months after achieving NH long-stay status (1-year continuous enrollment). Subsequently, qualitative interviews will focus on perspectives and experiences of people living with HIV aged 50+ with regard to facilitators and barriers to ART adherence while they were a long-stay NH residents. Recruited through two organizations, interviews will be semi-structured and analyzed with NVivo software using inductive and deductive theory building through grounded theory analysis and until thematic saturation is met. Triangulation using reconciliation and initiation techniques will address discordance between quantitative and qualitative findings. The results will provide nuance beyond administrative claims data by including social determinants and life course experiences (e.g., housing instability). This work will expand Dr Mui’s skills to include Medicaid SNP data, qualitative and mixed-methods research. She will be supported by four co-mentors with expertise in geriatrics, secondary data and qualitative and mixed-methods research, and health policy, in addition to formal training through Northeastern University and related resources. This career development award will complete Dr. Mui’s skills for researching hard to reach populations thereby helping establish her as an independent researcher and leader in HIV and aging. It will also be foundational to future R01 research to analyze healthcare utilization and health outcomes associated with ART adherence in the NH.