BACKGROUND AND AIMS: Fecal immunochemical test (FIT)-based colorectal cancer (CRC) screening is superior to the traditional binary fecal occult blood test. Its quantitative nature allows the investigator to choose a positivity threshold to match cost and endoscope capacity. The optimal threshold is still debated. BowelScreen, the Irish national colorectal cancer screening program, has a cut-off of 45 µg Hb/g feces, and in this study we investigated the impact of this threshold on pathology detected in round 2 in individuals who had a negative result for round 1 FIT (FIT1). METHODS: All individuals with a negative FIT1 result who completed a round 2 FIT (FIT2) 2 years later were included. Pathology outcomes for individuals who had positive FIT2 results were correlated with FIT1 levels. RESULTS: A total of 37,877 individuals had negative FIT1 results and completed FIT2. One thousand two hundred thirty (3.2%) had positive FIT2 results (702 men [57%], median age 69, age range 60-70 years). Quantitative analysis showed that at an FIT1 level <5 µg Hb/g feces, 2.3% had positive FIT2 results. At a higher cut-off of 40.1 to 45 µg Hb/g feces, 15.6% of individuals had positive FIT2 results. One thousand two (81.5%) underwent colonoscopy, with clinical outcomes in all cases. Three hundred fifty-one (35%) had normal colonoscopy results. The proportion of individuals with normal colonoscopy results decreased as FIT1 levels rose. Conversely, advanced pathology (CRC + high-risk adenomas) rates rose from 7% to 50% when FIT1 was <5 compared with 40.1 to 45 µg Hb/g feces, respectively. There were 51 screen-detected cancers in round 2 among individuals with negative FIT1 results (22 stage I, 12 stage II, 14 stage III, 3 stage IV). All 3 stage IV individuals had FIT1 results <20 µg Hb/g feces. CONCLUSIONS: Varying rates of pathology are observed in round 2 of a screening program based on the quantitative level of a negative round 1 FIT result when the positivity threshold is relatively high. A CRC rate of 5.1% within this group appears acceptable. Although patients with incurable cancer were observed, the positivity threshold to capture these cases within round 1 would have been so sensitive that it would create an unsustainable endoscopy referral burden.