Patients undergoing elective surgery for left-sided colon resection are asked to participate in this study: Every patient undergoes two types of examinations (endoscopy, CT scan) on day 3, 4 or 5 postoperatively. These two procedures are subject to investigation in terms of their accuracy (sensitivity, specificity) in detection of anastomotic leaks. The patients are followed up until day 42 postoperatively. There are no other specific tests or examinations within the study. Information and informed consent are obtained preoperatively.

On day 3, 4 or 5 postoperatively, all patients with colorectal resection and anastomosis will undergo a CT scan with intravenous (i.v.) and rectal contrast enema and a flexible endoscopy. Both are currently routine examinations. CT scan is performed before endoscopy because contrast enema clears the rectum and anastomotic area and no further cleaning measures are necessary. The timing of the CT scan and the endoscopy is chosen because early signs of anastomotic leak should be present and thus early therapeutic interventions before the development of peritonitis is possible. The standard treatment is not altered with the exception of the two described procedures. Due to infrastructural issues and weekends the CT scan and the endoscopy can be performed on either day (3,4,5 postoperatively).

The study is to start after Ethical approval. With a sample size of 400 patients, and an annual case load of 80-100 patients (minus drop outs of 10%), the study is planned to be finished (last patient processed) 31.12.2020. Data analysis and final processing will take another 6-8 months.

All patients with colorectal disorders planned for surgery are seen in the outpatient clinic of one the four staff surgeons performing colorectal resections. This appointment is at least 1 week before surgery. During the appointment, patients are fully informed on the disease, indication, peri- and postoperative process and risk of treatment. During this visit eligibility is analysed, and information as well as informed consent are obtained.

During the entire duration of the study, all serious adverse events (SAEs) that may be causally related to the study intervention are collected and documented in source documents. Reportable events are recorded in the case report form (CRF). Study duration encompassed the time from when the participant signs the informed consent until the last protocol-specific procedure has been completed, including a safety follow-up period.

The analysis entails tests for paired categorical (McNemar test) and continuous (Student's independent t-test, Wilcoxon test) data. Sensitivity, specificity, positive and negative predictive value and accuracy will be calculated with 95% confidence interval. The significance level is set to p<0.05.

Inclusion Criteria:

• Laparoscopic or open left sided colorectal resection with primary anastomosis (left hemicolectomy, sigmoid resection, anterior resection, segmental resection) and subtotal colectomy with anastomosis
• Male and Female patients >18 years of age
• Signed Informed Consent after being informed
• Elective surgery

Exclusion Criteria:

• Formation of a stoma during the initial operation
• For i.v. contrast CT scan: creatinine > 110 umol/l and/or glomerular filtration rate GFR < 45 ml/min, measured in the standard blood test on postoperative day 2
• Women who are pregnant. Women of childbearing age are regularly tested for possible pregnancy. Pregnancy tests are performed in the hospital at no charge
• Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc. of the participant
• Participation in another study with an investigational drug within the 30 days preceding and during the present study
• Emergency intervention

The study population entails all patients undergoing a colorectal left sided resection at the Spital Limmattal, Zurich, Switzerland according to the inclusion and exclusion criteria.