Effectiveness and acceptability of interventions to improve faecal immunochemical test (FIT) return in both asymptomatic (screening) and symptomatic populations: protocol for a systematic review of qualitative and quantitative evidence.

[INTRODUCTION] Colorectal cancer (CRC) is the fourth most common cancer in the UK and second leading cause of cancer-related deaths. The faecal immunochemical test (FIT) is a non-invasive home-based test used for both symptomatic assessment and population-based screening. However, approximately 30% of screening FIT kits and 10% of symptomatic FIT kits are never returned. Under-served populations, including ethnic minorities, socioeconomically deprived communities and those with mental health conditions, experience particularly low FIT return rates, contributing to health inequalities in CRC outcomes. This systematic review aims to synthesise evidence on the effectiveness and acceptability of interventions to improve FIT returns in both asymptomatic screening and symptomatic populations, with particular focus on under-served communities.

[METHODS AND ANALYSIS] We will conduct a systematic review of qualitative and quantitative evidence. We will search Scopus, MedLine via Ovid, CINAHL via Ebsco and Cochrane Central Register of Controlled Trials from September 2010 onwards, supplemented by reference screening and trial registry searches. Eligible studies will include randomised controlled trials, quasi-experimental studies, observational studies, qualitative studies, mixed-methods studies and implementation studies examining FIT interventions in screening or symptomatic populations. Two reviewers will independently screen search results for eligible studies. Data extraction will capture study characteristics, population demographics, intervention components and outcomes including FIT return rates, acceptability, feasibility and implementation factors. Quantitative data will undergo systematic tabulation and meta-analysis where appropriate, with narrative synthesis for heterogeneous studies. Qualitative data will be analysed using framework-based thematic analysis, mapping findings to both the theoretical domains framework and theoretical framework of acceptability. A mixed-methods synthesis will integrate quantitative and qualitative findings to identify intervention characteristics, implementation strategies and contextual factors associated with improved outcomes across different population groups.

[ETHICS AND DISSEMINATION] Ethics approval is not required as this systematic review will analyse published studies. Findings will be disseminated through peer-reviewed publication and conference presentations.

[PROSPERO REGISTRATION NUMBER] CRD420251111663.