Colorectal cancer (CRC) is one of the leading causes of death in Croatia. An average of 3600 cases are diagnosed and an average of 2100 people die from the disease every year. Since 2007, Croatia has invited every man and woman aged 50-74 to participate in the home testing screening programme every two years. Currently only around 36% of the invited request the test-kit and 25% complete the home testing procedure, far below the target of 40-60%.

The Croatian Institute of Public Health with technical support from World Health Organization (WHO) Regional Office for Europe is undertaking a mixed-methods research study with the aim to increase the completion of colorectal cancer home testing and improve our knowledge of the barriers and drivers to do so. The study has a quantitative and a qualitative component:

1. Quantitative: testing the introduction of a reminder letter to encourage people to respond to the initial invite to participate in the CRC home test program and test which elements of a reminder letter improve response rates.

   The quantitative component consists of a four-arm reminder letter randomized controlled trial (RCT) comparing no letter, standard letter, behaviorally informed letter, and behaviorally informed letter sent with a home testing kit to investigate the effectiveness of reminders in increasing uptake of home-testing for colorectal cancer. Recruitment and data collection for the reminder letter trial will be conducted utilizing the routine screening process and routinely collected screening data.

2. Qualitative: conducting in-depth interviews with people from the target population who did or did not respond to the CRC invite letter and reminder to better understand the barriers and drivers to participation.

The qualitative component consists of 24 in-depth interviews (IDIs) conducted with members of the target population to identify barriers and drivers to completing colorectal cancer screening home-testing. Data collection for IDIs will be face-to-face, using discussion guides, and will be audio recorded. The audio-recordings will then be analyzed using a rapid analysis approach based on by the modified Capability-Opportunity-Motivation-Behavior (COM-B) framework.

Inclusion Criteria:

• eligible to participate in the national colorectal cancer screening programme
• live in a study location (Split-Dalmatia or Zagreb City counties)
• did not respond to the cancer screening invite letter

Exclusion Criteria:

• participated in the colorectal screening programme in the last 24 months

The study utilizes the data that is routinely collected as part the Croatian National Screening Database, only specific approved members of the screening program are able to access and utilize this data. For this reason, it is currently not planned to share individual participant data.