A prospective, single-center, single-blind, randomized controlled study to compare the proportion of straight passage through the sigmoid colon without loop formation between conventional colonoscope passage, left colon water exchange colonoscope passage, and cap-assisted left colon water exchange colonoscope passage through the sigmoid colon

1. Patients are undergone screening, surveillance, or therapeutic colonoscopy at the Endoscopy department of Gastrointestinal endoscopy center of Huadong hospital affiliated to Fudan University.
2. Randomize patients into 3 interventional groups based on R script (version 4.4.1), including (1) Group 1: Conventional colonoscope passage, (2) Group 2: left colon water exchange colonoscope passage, (3) Group 3: cap-assisted left colon water exchange colonoscope passage.
3. Collecting variables which consist of primary and secodary outcomes.

Inclusion Criteria:

• aged 18-80 years
• sedation colonoscopy

Exclusion Criteria:

• History of colorectal surgery
• Clinical suspicion of, or pathologically confirmed, colorectal cancer
• Presence of any "alarm" symptom or sign-hematochezia, melena, unexplained anemia or weight loss, palpable abdominal mass, or a positive digital rectal examination
• Pregnancy or lactation
• Known or suspected gastrointestinal obstruction
• Diagnosed inflammatory bowel disease
• Use of antiplatelet or anticoagulant therapy (e.g., aspirin, warfarin) within 7 days before colonoscopy or any documented coagulation disorder
• Current enrolment in another clinical study or participation in any clinical trial within the previous 60 days
• Requirement for emergency colonoscopy
• Any other condition that, in the investigators' judgment, renders the patient unsuitable for inclusion in the study

All collected IPD

1 year after completion of this study

All researchers can require the original data from principal investigator of this study (Email address: arctg4@163.com)