1 ABSTRACT The National HIV Strategy encourages expansion of HIV testing to community health settings, emphasizing it is critical that all people, particularly high incidence groups like people who inject drugs (PWID), know their HIV status for rapid initiation of HIV treatment or potential initiation of PrEP. Despite recommendations from CDC that PWID be tested for HIV every year, only 55% report receiving an HIV test in the previous 12 months. Injection drug use (IDU) continues to be a leading cause of HIV infection and PWID remain a high priority population under Ending the HIV Epidemic (EHE) with 11% of new HIV infections and 66% acute HCV infections attributable to IDU in 2020. In recent years, there has been a national increase in IDU-associated HIV and hepatitis C (HCV) outbreaks, driven primarily by the ongoing overdose crisis, creating an obstacle to meeting EHE goals. While the majority (87%) of syringe services programs (SSPs) offer HIV testing, it is estimated that participant-level uptake is only 17%, highlighting a significant practice gap regarding how and under what conditions SSPs are offering HIV testing. SSPs face a myriad of organizational-level barriers to implementing HIV testing, such as lack of financial resources, lack of training on best practices, staff turnover, and organizational capacity/policies. Our team developed, refined, and pilot tested the ACCESS implementation strategy—a multicomponent, multi- phase, organizational-level intervention focused on improving the SSP organizational capacity to implement HIV/HCV testing best practices through funding and practice facilitation. We propose a hybrid Type II effectiveness-implementation cluster randomized trial of 32 SSPs. We will create a cross-sector implementation team to facilitate the ACCESS implementation strategy in EHE jurisdictions and locales with increased vulnerability to HIV outbreaks. Selected SSPs will then be randomized to receive the ACCESS strategy (i.e., funding and peer external facilitation), or an information control condition that contains a link to the CDC Compendium website and information regarding nationally available resources to support implementation. The primary outcome of the study will be HIV/HCV testing reach during the intervention period (12 months) compared to control sites. The aims of the study are: 1a) to evaluate the effectiveness of the ACCESS strategy on participant-level uptake of HIV/HCV testing across 12 months; 1b) to examine the effect of routine opt-out HIV/HCV testing on equitable HIV/HCV testing uptake and diagnosis across 12 months; 2) to examine the effectiveness of our ACCESS implementation strategy on site-level adoption of opt-out testing and implementation fidelity; 3) to assess the cost-effectiveness of the ACCESS strategy in increasing uptake of HIV/HCV testing; and 4) to examine the sustainability of opt-out HIV/HCV testing at SSPs across 24 month follow-up. The cost-effectiveness analysis and mixed-methods im...