Effect of long-term adherence on faecal immunochemical test positivity rate, positive predictive value and detection rate in organised population-based colorectal cancer screening

BACKGROUND: Long-term adherence and results with faecal immunochemical test (FIT)-based colorectal cancer (CRC) screening are poorly characterised. OBJECTIVE: To characterise adherence and results through seven rounds in an organised biennial FIT-based CRC screening programme. DESIGN: We determined per-round FIT-completion, FIT-positivity, CRC and high-risk-CRC precursor positive predictive values (PPVs) and CRC detection/1000-FIT-participants for all invitees versus an adherent cohort (entry 50-51 years; 66-100% rounds completed) versus comparable-age first-ever screenings from 2010 to 2023. Joinpoint and multivariable logistic regression analyses identified trends. RESULTS: Adherence was consistent, frequent, occasional, infrequent and never (defined as 100%, 66-99%, 33-65%, 1-32%, 0% of rounds offered) in 29.2%, 8.6%, 11.5%, 4.5% and 46.2% of 2.81 million individuals, respectively. In both the all-invitee population and the adherent cohort, the first round yielded the highest FIT positivity (5.8%, 4.4%), PPVs for CRC (5.1%, 3.3%) and high-risk precursors (20.4%, 13.1%), and CRC detection rates (2.65, 1.30 per 1000 participants), respectively. Beyond three rounds, outcomes stabilised at levels substantially lower than those observed in first-time screeners of the same age (eg, CRC-PPV in seventh round: 1.6-2.2% at median age 62-65 vs 6.6% for new screeners aged 62-63). Colonoscopy completion after a positive FIT was high (87.3%). CONCLUSION: After an initial round with the highest FIT-positivity and detection rates, screening outcomes stabilise at lower levels reflecting neoplasia removal and subsequent selection of a lower-risk population. Because detection rates remain clinically relevant even in an adherent cohort, early screening cessation after a sequence of normal biennial FITs is not justified.