Colorectal cancer is a leading cause of mortality in Catalonia. Although early detection programs using the fecal immunochemical test (FIT) are effective in reducing both incidence and mortality, their success relies on high population participation. Currently, in the Vallès Occidental region, the participation rate stands at 42%, which is below the 65% minimum recommended by European health authorities. The objective of this randomized controlled trial is to evaluate whether sending a reminder text message (SMS) is an effective tool to increase participation in the screening program. The study will include 10,084 participants aged between 50 and 69 years. Half of the participants will receive a reminder SMS five weeks after their initial invitation, while the other half will follow the standard of care involving postal letters. Researchers anticipate that this strategy will not only increase the number of individuals undergoing screening but also shorten the response time and reduce the need for sending postal reminders

This study is a pragmatic, randomized, parallel-group clinical trial with a 1:1 allocation ratio, conducted within the framework of the Colorectal Cancer Early Detection Program (PDPCCR) in the Vallès Occidental region. The trial is coordinated by the Cancer Screening Unit of the Consorci Corporació Sanitària Parc Taulí.

Operational Flow and Randomization: The study follows a sequential recruitment process. At Week 0, the entire target population receives a standard invitation letter according to the program's usual standard of care. For individuals who have not participated by Week 5, a computer-generated randomization sequence (Microsoft Excel) is applied weekly to assign them to either the intervention or control group. This weekly approach ensures the intervention is timely and integrated into the program's existing administrative workflow.

Intervention Logistics: The Intervention Group receives a one-way SMS reminder sent via the 'Alhora' platform, the secure institutional messaging system used by the screening program. The content of the SMS was previously optimized and validated through a qualitative pilot study involving semi-structured interviews to ensure high readability and cultural acceptability among the target population. Non-participants in both groups receive the standard postal reminder letter at Week 6.

Statistical Analysis Plan: The analysis will be conducted on both an intent-to-treat (ITT) and per-protocol basis. Descriptive statistics will report frequencies and medians with interquartile ranges (25th-75th percentiles). Bivariate analysis will utilize Chi-square tests for participation rates and the Mann-Whitney U test for response time differences. To estimate the effect size, Relative Risks (RR) with 95% confidence intervals will be calculated. Furthermore, a multivariate binary logistic regression will be performed using Jamovi software to adjust for potential confounders such as sex, age, geographic area, and history of previous participation

Inclusion Criteria:

• Women and men aged 50 to 69 years.
• Residents of the Vallès Occidental area.
• Individuals invited to the Colorectal Cancer Screening Program (PDPCCR) who have not participated within 5 weeks of the initial invitation.

Exclusion Criteria:

• Personal history of colorectal cancer.
• Inflammatory bowel disease (IBD)
• Colorectal polyps requiring specific clinical follow-up.
• Hereditary polyposis syndromes.
• High-risk family history of colorectal cancer: a first-degree relative diagnosed before age 50, or two or more first-degree relatives at any age.
• Severe morbidity that precludes the performance of a colonoscopy in the event of a positive test result.

Restricted Access and Security: The protocol stipulates that data will be stored on secure hospital servers with access strictly limited to authorized research staff through encryption systems and secure passwords.

Privacy and Confidentiality: The use of personal identifying data is strictly limited to the technical execution of the intervention (SMS delivery). The research team has implemented measures to ensure the anonymization of the analytical database.

Legal Compliance: The study is governed by Regulation (EU) 2016/679 (GDPR) and Organic Law 3/2018 on the Protection of Personal Data, which imposes legal restrictions on the transfer of individual data outside the consented framework for this specific research.

Due to the nature of the behavioral intervention (sending an SMS reminder), masking of participants and investigators is not possible. However, to minimize bias, the primary outcome measure (participation) is objective, as it is automatically determined by the screening program's information system when the participant's sample is analyzed in the laboratory. Furthermore, statistical analysis will be performed on an anonymized database where the analyst will use unique identification codes dissociated from personal data.