Background Digital interventions designed to expand access to sexually transmitted and blood-borne infection (STBBI) testing are promoted to advance equitable health services. Yet many remain pilot projects, limiting their population-level impact. We examined factors influencing the sustainability of GetCheckedOnline, British Columbia’s digital STBBI testing service and described how these factors interact over time during transition beyond piloting and into routine operations.Methods We conducted a qualitative instrumental case study guided by the Dynamic Sustainability Framework. Purposive sampling captured perspectives across intervention, organizational, and system levels. Semi-structured interviews and one focus group were conducted with 28 health systems partners between February and June 2025. Reflexive thematic analysis was used, with attention to temporal shifts across pilot, scale-up, and ongoing operations.Results Four interconnected themes characterized GetCheckedOnline’s sustainability from pilot to scale, demonstrating how early enabling conditions became constraining as post-COVID testing demand increased, laboratory costs rose, and fiscal pressures intensified. First, values-driven urgency acted as both catalyst and constraint: an equity mandate fueled rapid expansion but limited planning for governance, infrastructure, and funding, and early design choices (e.g., manual results entry to support non-nominal testing) created operational complexity. Second, early implementation through flexible governance structures became misaligned at scale, where clearer ownership and accountability mechanisms were required to support system integration. Third, informal relational supports and team resilience were critical to early success, as these enabled progress through trusted partnerships and individual commitment; yet, these placed hidden burdens on staff to manage processes that were not considered as standard operations. Finally, ambiguous system structures which allowed early flexibility became constraints, as reliance on a single laboratory partner able to meet privacy requirements, project-based funding pathways, and limited mechanisms for transitioning pilots into operations impeded full integration despite the service’s demonstrated value.Conclusion GetCheckedOnline’s evolution highlights a patterned inversion during scale up, when early enabling conditions became structural constraints when lacking formal transition mechanisms. For equity-oriented digital services, deliberate pause points and institutionalization of governance, funding, and technical systems are critical for scale. By articulating this inversion dynamic, the study contributes to implementation science and offers insights for jurisdictions scaling similar innovations.