Patients with colorectal adenomas are suggested to receive polypectomy. A substantial number of adenomas may be missed in patients with polyps found by previous colonoscopy. And water exchange (WE) method may increase the detection of missed adenoma compared with traditional air insufflation (AI) colonoscopy. The investigators aim to investigate whether water exchange colonoscopy method, compared with air insufflation method, can improve the detection of missed adenomas in patients undergoing selective polypectomy.

For WE method, the air pump was turned off for the full duration of insertion to avoid inadvertent air insufflations and colon elongation. Residual air in the lumen was suctioned to minimize angulations at flexures. Water at 37°C was infused with a pump (Olympus) through the biopsy channel to confirm correct tip orientation for scope advancement. The infused water was removed predominantly by suction when the colonoscope was smoothly advanced during the insertion phase, and turbid luminal water due to residual feces was exchanged by clean water until the lumen was clearly visualized. Occasionally, if it was difficult to determine whether the colonoscope tip was in the cecum, air was allowed to be insufflated for observation. If the position of the scope tip was confirmed to be not in the cecum, insufflated air would be removed by suction and the WE method would be continuously used until successful intubation.

For the AI method, water was not used, and air was insufflated during insertion.

Air was insufflated to distend the lumen for inspection and biopsy or polypectomy during withdrawal for both methods. Polyps will be removed by forceps biopsy (polyps size <3mm), cold snare technique (3-6mm) or endoscopic mucosal resection (size≥6mm).

Inclusion Criteria:

• Age 18-80 patients undergoing selective polypectomy

Exclusion Criteria:

• Polyps found more than 6 months
• Patients with polyposis syndrome or hereditary nonpolyposis colorectal cancer
• Patients with history of inflammatory bowel disease
• Patients with planning to undergo Endoscopic Submucosal Dissection(ESD)
• Patients not undergoing standard bowel preparation
• Patients with solid feces in the last stool after bowel preparation
• Patients considered to be high risk for bleeding during Endoscopic mucosal resection (EMR), e.g. using antiplatelet drugs (clopidogrel) within 5 days before the current colonoscopy
• Hemodynamically unstable
• Pregnant women
• Unable to provide informed consent