RESUMEN
This study aimed to test whether participation in colorectal cancer (CRC) screening can be increased by combining the standard invitation procedure with a pre-notification and/or an extra reminder. In a non-blinded randomised controlled trial nested in a population-based CRC screening programme employing the faecal immunochemical test, Group I received a three-staged invitation procedure (pre-notification, invitation and one reminder), Group II received a three-staged invitation procedure (invitation and two reminders) and Group III received a four-staged invitation procedure (pre-notification, invitation and two reminders). The control group received the invitation and one reminder (usual procedure). A total of 59,041 participants were included in the analyses. Overall participation rates increased from 66.9% in the control group to 69.8% in the four-staged invitation procedure corresponding to an increase in overall participation rate of 2.9% (95% CI: 1.8 to 4.0). In the age group 50-59 years, the four-staged invitation procedure increased the participation rate by 4.0% (95% CI: 2.4 to 5.6). An extra reminder increased participation with 2.7% (95% CI: 1.1; 4.2) for males compared to 1.1% (95% CI: -0.3; 2.5) for females. In conclusion, the four-staged invitation procedure was the most effective invitation procedure indicating that multiple invitation procedures are most effective, especially in the youngest age group. If a three-staged invitation procedure is applied, a second reminder should be preferred over a pre-notification. Trial registration The project was registered at ClinicalTrials.gov on 26 February 2020 and patient enrolment began in August 2020. ClinicalTrials.gov Identifier: NCT04292366.