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.

Background The Basque Health Service (Spain) approved the Colorectal Cancer (CRC) screening programme in 2008; targeted at residents (50 to 69 years old) by biennial faecal immunochemical test and colonoscopy in positive cases. Total coverage was reached in 2014. Despite our high participation rate (72.3%), some people have never been screened, probably due to several factors (socio-economic factors, lifestyles, comorbidities, health preventive actions, etc.). The identification of these factors could be crucial in order to improve the results and impact of the programme. Methods Cross sectional descriptive study. All invited population in a complete round between 2015 and the first trimester of 2017 were included. Data base was linked to registered health risk factors, preventive activities managed by Primary Care and socioeconomic deprivation index. Univariate and multivariate analyses were performed by SPSS 23.0. Results 515,388 people were invited and 71.9% participated. Non-participation is significantly (p ≤ 0.001) related to: being men (OR = 1.10); younger than 60 (OR = 1.18); smoker (OR = 1.20); hypertensive (OR = 1.14) and diabetic (OR = 1.40), as well as to having a severe comorbidity (OR = 2.09) and very high deprivation (OR = 1.15). Low use of Primary Care services (OR = 2.39) and inappropriate control of risk factors also increase the likelihood of not participating. Conclusions Several factors addressed in Primary Care influence the participation in the CRC Screening Programme. Its results and impact (decreasing incidence and mortality) could be improved if individual and socioeconomic factors are taken into account to increase participation. Primary care and social networks are key in further planned interventions. Key messages Decreasing inequalities in the screening participation is one of the most important objectives in this Public Health strategy. Primary Care plays a key role in all the screening programmes’ implementations.