Impact of time between faecal immunochemical tests in colorectal cancer screening on screening results: A natural experiment.

Repeated rounds of faecal immunochemical testing (FIT) for occult blood is a common method for screening for colorectal cancer (CRC). However, the time interval between FIT rounds is not thoroughly investigated. In a CRC screening trial in South-Eastern Norway, individuals were invited for biennial FIT between 2012 and 2019. The positivity threshold was >15 mcg haemoglobin/g faeces (mcg/g). Due to organizational challenges, the interval between screening rounds randomly varied between 1.5 and 3.5 years, forming a natural experiment. We investigated the detection rate of CRC and advanced neoplasia (AN: CRC or advanced adenoma) at the subsequent round (FIT ), according to the faecal haemoglobin concentration (f-Hb) at the initial screening round (FIT ), and time between the two screening rounds. 18 522 individuals with negative FIT who attended FIT were included in this study. 245 AN were detected at FIT , of which 34 were CRC. The CRC detection rate at FIT for participants with FIT  = 0 mcg/g was 0.09% while it was 0.28% for participant with 0 > FIT  ≤ 15 mcg/g; odds ratio (OR) 3.22, 95% CI 1.49-6.95. For each 3 months' increment between FITs, the OR for detecting CRC was 1.33 (95% CI 0.98-1.79), while the OR was 1.13 (1.02-1.26) for AN. Individuals with FIT -value of 0 mcg/g, had a lower AN detection rate compared with participants with 0 > FIT  ≤ 15 mcg/g, irrespective of time between tests. Although CRC and AN detection rates increase with increasing time interval between FITs, individuals with undetectable f-Hb at first screen have substantially lower risk of CRC at the next screening round compared with individuals with detectable f-Hb.