The diagnostic accuracy of the faecal immunochemical test for the detection of early-onset colorectal cancer: an age-stratified analysis in South West England.

Background: The incidence of early-onset colorectal cancer (EOCRC) is rising rapidly, with diagnoses typically occurring at a more advanced stage than late-onset CRC. In the absence of screening for younger patients, diagnosis relies on symptomatic presentation. The faecal immunochemical test (FIT) is a diagnostic triage tool for patients presenting with clinical features of CRC in primary care, though its performance in individuals under 50 years is not well established.

Methods: A cohort of 38,117 symptomatic patients aged 18-49 years in upper South West England underwent FIT in primary care between 01/01/2021 and 10/07/2023. A FIT result of ≥10 µg Hb/g faeces was considered positive. In the same region, 528 EOCRC diagnoses were recorded between 01/01/2021 and 10/10/2024.

Results: Of the 528 EOCRC patients, 105 (20%) underwent FIT in the year before diagnosis. The sensitivity of FIT was 92.4% (95% CI 85.5-96.7%), specificity was 88.5% (88.2-88.8%), positive predictive value (PPV) was 2.2% (1.8-2.6%), and negative predictive value was 100% (100-100%). PPVs decreased in younger age groups (18-29, 30-39, 40-49 years).

Conclusions: FIT performs excellently for patients aged 40-49; however, it may not be used optimally in patients <40 years. A more targeted strategy is needed to guide investigation in younger patients.