COLSCREEN
The research cohort (COLSCREEN study) was recruited among individuals that participated during 2016-2020 in the ongoing population-based CRC screening program overseen by the Catalan Institute of Oncology in L'Hospitalet del Llobregat, Barcelona, Spain. This program invites men and women between the ages of 50 and 69 to partake in the immunochemical fecal occult blood test (FIT). In the event of a positive FIT result (= 20 µg Hb/g feces), the participants are then recommended to undergo colonoscopy. Participants of the COLSCREEN study (N=997) were invited to participate after receiving a positive FIT result. Since CRC diagnosis is rare in screening, this cohort includes patients diagnosed clinically from the CRC Functional Unit (N=45). Furthermore, a subset of participants with a negative FIT is also included (N=140). All of them underwent a colonoscopy and participants were categorized based on the risk stratification proposal by Castells et al. after careful review of the colonoscopy and histopathology reports. All participants who agreed to take part of the COLSCREEN study provided written informed consent and donated a fecal and blood sample at recruitment (samples obtained before colonoscopy) and answered an epidemiological questionnaire. Clinically diagnosed CRC patients usually underwent a colonoscopy before recruitment, and the fecal samples were obtained prior surgery. In the present study, we excluded those participants that reported having used antibiotics or probiotics one month before sampling.