Interval colorectal cancers after negative faecal immunochemical test in the New Zealand Bowel Screening Pilot.

Objective: Evaluate the diagnostic performance of faecal immunochemical test (FIT), identify risk factors for FIT-interval colorectal cancers (FIT-IC) and describe long-term outcomes of participants with colorectal cancers (CRC) in the New Zealand Bowel Screening Pilot (BSP).

Design: From 2012 to 2017, the BSP offered eligible individuals, aged 50-74 years, biennial screening using a quantitative FIT with positivity threshold of 15 µg haemoglobin (Hb)/g faeces. Retrospective review of prospectively maintained data extracted from the BSP Register and New Zealand Cancer Registry identified any CRC reported in participants who returned a definitive FIT result. Further details were obtained from hospital records. FIT-ICs were primary CRC diagnosed within 24 months of a negative FIT. Factors associated with FIT-ICs were identified using logistic regression.

Results: Of 387 215 individuals invited, 57.4% participated with 6.1% returning positive FIT results. Final analysis included 520 CRC, of which 111 (21.3%) met FIT-IC definition. Overall FIT sensitivity for CRC was 78.7% (95% CI=74.9% to 82.1%), specificity was 94.1% (95% CI=94.0% to 94.2%). In 78 (70.3%) participants with FIT-IC, faecal Hb was reported as undetectable. There were no significant associations between FIT-IC and age, sex, ethnicity and deprivation. FIT-ICs were significantly associated with proximal tumour location, late stage at diagnosis, high-grade tumour differentiation and subsequent round screens. Median follow-up time was 74 (2-124) months. FIT-IC had significantly poorer overall survival.

Conclusion: FIT sensitivity in BSP compared favourably to published data. FIT-ICs were more likely to be proximal tumours with poor long-term outcomes. Further lowering of FIT threshold would have minimal impact on FIT-IC.