Background: A faecal immunochemical test (FIT) result ≥ 10 μg/g is recommended in the UK to triage patientswith symptoms of colorectal cancer (CRC) in primary care for urgent cancer investigation. The COLOFIT model com-bining FIT results with demographics and blood tests was developed to reduce the proportion of people referredwithout CRC. This study aims to externally validate the COLOFIT using data from Oxford University Hospitals (OUH).
Methods: FITs requested by GPs between January 2017 and February 2024 were extracted from the OUH ClinicalData warehouse. Adults with COLOFIT predictors and 180-day follow-up for CRC were included. External valida-tion of the COLOFIT equation was conducted overall and for six independent time periods. Risk score thresholdswhere the model captured the same number of cancers as FIT ≥ 10 μg/g were estimated to understand the numberof urgent referrals avoided.
Results: A total of 51,477 individuals (659 CRC) were included; 6194 (12%) had FIT ≥ 10 μg/g. FIT positivity and test-ing volume increased over time, associated with a gradual change from testing lower-risk patients to including thosewith higher-risk symptoms. COLOFIT was poorly calibrated overall (observed/expected [O/E] ratio 1.52 with calibrationslope 1.05), but calibration improved over time (up to O/E ratio 1.09 with calibration slope 1.05). COLOFIT reducedreferrals by 8% overall without missing colorectal cancers compared to FIT ≥ 10 μg/g, but this varied from 23% reduc-tion to 2% increase depending on the period evaluated.
Conclusions: The potential benefit of COLOFIT varied depending on FIT testing rates, the proportion of FIT ≥ 10 μg/g,and the symptoms in the tested population. Adopting COLOFIT into current clinical practice demands, therefore, FITpositivity of at least 17% and CRC rates within 1.3–1.6%. Further validation in local and different populations wouldalso be of significant value and help to maximise COLOFIT’s ability to improve diagnostic pathways.