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Background Colorectal Cancer (CRC) Screening started in 2009 inviting biennially to carry out an immunochemical quantitative occult blood test (FIT). Overall coverage was reached in 2014 The aims of this study were to determine the main features of Interval Colorectal Cancer (CRC), related to FIT (IC_FIT= false negatives of the FIT) on the population-based screening Programme (BCSP) Methods All invitees from 2009-2016 were included in the study and with a follow-up by the Screening Coordination Centre according to the European Guidelines. Specific software was developed for the BCSP and able to be linked to medical records and cancer registries. Variables included: age at invitation, sex, type of participant (initial, regular, and irregular), TNM stage, and location (C-18.0-C-20.0), FIT threshold. Sensitivity Specificity and LR ratios were performed throughout all the period for FIT test. SPSS 23.0 Results 1,386,538 valid invitations were registered between 2009 and 2016. With a high participation rate (69.3%) and a total of 926,379 FIT negatives (94.1%). 444 CI_FIT were found before the next invitation to FIT. Most in men 60.7% (n = 278), higher on the oldest age group (60-69 years of age). 33.4% where found on regular participants on the screening. 82.1% of IC_FIT in previous rounds the FIT threshold was <10µHb/ g. Most of the IC_FIT 54.1% found on advanced Stage of the tumor, and located on the right colon (44.4%), but ringing a bell the 30.4% located on Rectum. The cumulative incidence for IC-FIT was 3.52 (IC95% 3.21-3.87) x 10,000 negative cases. Sensitivity for FIT was lower in women than men (87.7; CI 95% 85.8-89.4 vs 88.9%; CI95% 87.6-90.1) but not significant. Cumulative incidence for IC-FIT is significantly higher in men than women Conclusions The BSCP in the Basque Country shows a low incidence of Interval Cancer due to FIT. However, right colon and rectum location must be taken into account in further studies, in order to decrease the IC_FIT Key messages Participation on a BSCP is essential to decrease Incidence and mortality despite the IC_FIT (false negatives of the test). Most of the persons (82.1%) of IC_FIT group in the previous round of participation, the FIT threshold was very low.

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