Background: Colorectal cancer (CRC) screening with fecal immunochemical testing (FIT) has been offered nationwide in Denmark since 2014. When FIT results are positive, screening participants are referred for a colonoscopy. However, further screening of individuals with a previous colonoscopy prior to screening enrolment might be redundant. An earlier colonoscopy that did not detect CRC may be associated with a reduced risk of CRC; however, evidence is lacking. We therefore investigated the effect of pre-screening colonoscopies on outcomes of participants in the Danish CRC screening program.

Methods: We conducted a population-based cohort study of all first-time participants in the Danish CRC screening program between Jan. 1, 2014, and Dec. 31, 2021. Using registry data, we identified participants with a previous colonoscopy. Outcomes included FIT screening results, as well as colonoscopy findings following a positive FIT, categorized as: no finding (i.e., a clean colon), low-risk adenomas, medium-risk/high-risk adenomas, or CRC. Risk of a post colonoscopy CRC among participants with no finding on their follow-up colonoscopy was also estimated. We computed prevalence and prevalence differences (PD), as well as 3- and 7-year cumulative incidence proportions (CIP), together with crude and adjusted hazard ratios (HRs and aHRs).

Results: We included a total of 1,594,130 participants who underwent FIT screening in the study. Among these, 215,381 (13.5%) had a previous colonoscopy, while 1,378,749 (86.5%) had no previous colonoscopy. The prevalence of FIT-positive results, medium-risk/high-risk adenomas, and CRC were 6.6%, 26.7%, and 4.9% respectively. Compared with participants with no previous colonoscopy, a history of colonoscopy conducted ≤2, 2.1-5, 5.1-10, 10.1-15, and >15 years prior to FIT screening was associated with a higher prevalence of FIT-positive results (PD in percentage points: 1.9 [95% CI: 1.7 , 2.1]; 1.3 [95% CI: 1.1 , 1.5]; 1.3 [95% CI: 1.1 , 1.6]; 1.5 [95% CI: 1.1 , 1.8], and 2.4 [95% CI: 1.9 , 2.9]), respectively. However, a history of colonoscopy was associated with a lower prevalence of medium-risk/high-risk adenomas (PD in percentage points: -18.3 [95% CI: -19.2 , -17.5]; -15.2 [95% CI: -16.0 , -14.5]; -12.4 [95% CI: -13.2 , -11.5]; -9.3 [95% CI: -10.7 , -7.9], and -7.1 [95% CI: -8.8 , -5.5] ), respectively. Additionally, a history of colonoscopy was also associated with a lower prevalence of CRC (PD in percentage points: -4.0 [95% CI: -4.4 , -3.7]; -3.8 [95% CI: -4.1 , -3.5]; -3.2 [95% CI: -3.6 , -2.9]; -2.6 [95% CI: -3.2 , -2.0], and -1.9 [95% CI: -2.6 , -1.1]), respectively.

Conclusion: Pre-screening colonoscopies were associated with a higher proportion of FIT-positive tests and a lower risk of adenomas and CRC following a subsequent diagnostic colonoscopy.

Frederikke Schønfeldt Troelsen

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