Diagnostic Yield of Fecal Immunochemical Test for Advanced Colorectal Neoplasms in Adults Under 50: A Single-Center Cohort in Taiwan

Background/Objectives: The incidence of early-onset colorectal cancer is increasing worldwide. The fecal immunochemical test (FIT) is widely used for screening adults aged 50 and older, but its performance in younger individuals is less understood. Methods: We retrospectively analyzed 202,676 FITs from asymptomatic adults aged 18–49 between 2011 and 2025. FIT results, age categories, and follow-up colonoscopy findings were evaluated. Results: The FIT positivity rate was 4.7%. Among 1973 FIT-positive individuals who underwent colonoscopy, 5.9% had advanced adenoma or sessile serrated lesion and 1.3% had invasive cancer. A total of 143 advanced neoplasms (ANs) were detected, with prevalence increasing with age. Most ANs (79.1%) occurred in those aged 40–49. The prevalence of ANs was higher in the 45–49 than in the 40–44 age group (49.0% vs. 30.1%; OR 1.55, 95% CI 1.04–2.32) and higher in the 40–44 than in the 35–39 age group (30.1% vs. 10.5%; OR 2.20, 95% CI 1.20–4.02). Conclusions: The diagnostic performance of FIT in individuals under 50 years is comparable to that observed in the older population. Given the age-related rise in the prevalence of ANs in young adults, several countries have lowered the screening age to 45 years, and extending screening to individuals aged 40 years may be warranted.