Colorectal Omics and OfCS Proteoglycans

The prospective cohort study will include 1,000 patients undergoing elective colorectal resections or colostomy reversal at two colorectal centers in Denmark. Repeating study blood samples will be collected on each postoperative day (POD) 1-4 or until discharge. If the participants are diagnosed with cancer, blood sampling is planned 26-35 days after the index procedure, and after one, two, and three years, a tumor biopsy will be taken from the fresh specimen in the operation theatre. Analyses of blood plasma and tissue for oncofetal chondroitin sulfate (ofCS) proteoglycans, proteomics, and metabolomics will be performed on those repeating blood samplings to: * To investigate whether metabolomics, proteomics, and ofCS techniques can identify new biomarkers where charges in plasma levels can predict or detect subclinical AL (primary outcome) and other major postoperative complications after colorectal surgery and prediction of 90-day and three-year mortality. * To investigate whether the APOE genotype is associated with the risk of AL and other major postoperative complications and long-term outcomes, i.e., recurrence and mortality, after colorectal surgery. * To examine whether metabolomics and proteomics can identify new biomarkers predicting recurrence after colorectal cancer resections. * To identify other potential biomarkers that might enable early cancer diagnosis. * Whether proteoglycans can be used as a diagnostic test with a high degree of separability to identify tumor markers that are usable in a clinical setting (primary outcome). Participants with colorectal cancer will be compared with a control group of participants with benign conditions. * Whether the level of proteoglycans measured correlates to the tumor load. * Whether the proteoglycans and proteomics detected in plasma are presented in tumor tissue from the resected specimen. * Whether the proteoglycans detected can be used as tumor markers with a high degree or measure of separability for monitoring recurrence after colorectal cancer in a clinical setting (primary outcome). Postoperative and follow-up data will be collected prospectively for the electronic health records.