Varying faecal immunochemical test positivity thresholds impacts colorectal cancer detection

BACKGROUND: Colorectal cancer (CRC) is screened for by the faecal immunochemical test (FIT) in Ireland. The FIT positivity threshold in Ireland is 45 micrograms (µg) of haemoglobin per gram (g) of stool. FIT positivity thresholds influence both demand for screening colonoscopies and CRC detection rates through screening. This study aimed to assess the impact of varying FIT positivity thresholds on CRC detection rates through screening in Ireland. METHODS: This study used data from BowelScreen - The National Bowel Screening Programme, to retrospectively assess the impact of altering the FIT positivity threshold, adjusting it to correspond with neighbouring European countries, on CRC detection rates through screening in Ireland between 2013 and 2021. All analyses were conducted on Stata 17. RESULTS: 1,552 individuals were diagnosed with CRC in Ireland after a positive FIT performed through BowelScreen between 2013-2021. Adopting the same FIT positivity thresholds as the Netherlands (47µg/g), Scotland (80µg/g), England (120µg/g) and Wales (150µg/g) would have resulted in 21 (1.4%), 262 (17.1%), 458 (29.8%) and 556 (36.2%) fewer diagnoses of CRC through BowelScreen during this period, respectively. Additionally, increasing the positivity threshold to correspond with these countries would have resulted in between 1.7-41.9% fewer stage 1, between 1.0-30.9% fewer stage 2, between 1.4-32.9% fewer stage 3 and between 0-5.6% fewer stage 4 CRCs detected. CONCLUSIONS: Changing the FIT positivity threshold in Ireland to correspond with that of other neighbouring European countries would have reduced the detection of CRC through screening to varying degrees. The detection of stage 1 CRCs would have been most greatly reduced, but up to almost a third of all stage 2 and stage 3 CRCs would not have been detected if some alternative, higher positivity thresholds were adopted. KEY MESSAGES: • Increasing the FIT positivity threshold in Ireland will need careful consideration to keep the optimum positive predictive value of a colonoscopy. • Lower stage 1-3 CRC detection rates may occur as a result.