Abstract Background Online postal self-sampling (OPSS) for sexually transmitted infections and blood borne viruses (STI/BBVs) has been introduced in several countries, because it may lower costs, and increase access for service users. There are gaps in the evidence on the equitable delivery to underserved populations beyond men who have sex with men, the implementation and maintenance of programmes and its impact on wider health systems. This study synthesised evidence from the ASSIST research programme, which evaluated the implementation, equity, impacts and economic consequences of OPSS in England (2015–2022), with a focus on understanding how and why unintended consequences may emerge in other contexts. Methods A synthesis using a realist logic of analysis was undertaken across multiple sources of evidence from ASSIST workstreams. Context-mechanism-outcome configurations were developed from a systems perspective. They were iterated through comparisons with initial programme theory and through feedback obtained from sexual health providers, funders, researchers and service users. Results In England, OPSS appears convenient and achieved higher testing uptake for lower costs per diagnosis. However, there were unintended consequences, principally: (1) difficulties in containing demand for OPSS could result in higher total costs than planned; (2) the introduction of OPSS affected wider sexual health system sustainability, particularly when the context changed; (3) inequalities in sexual health may widen after the introduction of OPSS in part due to missed presentation opportunities. These challenges prompted adaptive responses in health systems, leading to a rebalancing between OPSS and clinic-based services. Conclusions While the introduction of OPSS in the context of reducing sexual health budgets offers some clear advantages to entirely clinic-based sexual health services, excessive demand and difficulties in access and use risk destabilising sexual health systems and worsening inequalities. Strategic rebalancing of OPSS and clinic-based provision is essential to mitigate unintended effects and ensure equitable, resilient service delivery. Protocol https://doi.org/10.1136/bmjopen-2022-067170
Sheringham et al. (Mon,) studied this question.