The primary aim of this study is

- to explore the usefulness of re-examination and retroflexion on adenoma miss rate (AMR) in the proximal colon.

Other aims include to explore the data below when re-examination or retroflexion is used.

• Adenoma detection rate, ADR
• Polyp miss rate, PMR
• Polyp detection rate, PDR
• Withdrawal time, WT

AMR = number of adenomas missing during the first examination/ total number of adenomas in both examinations = number of adenomas detected only in the second examination/(number of adenomas detected during the first examination + number of adenomas detected only in the second examination)

Inclusion Criteria:

• Patients between 45 and 80 years who undergo colonoscopy examination for screening, and who receive primary screening and get positive result

Exclusion Criteria:

• Pregnant female patients
• Patients received colonoscopy in the past 5 years
• Patients who have history of colorectal cancer, colorectal polyposis, inflammatory bowel disease or heredity colorectal neoplasm syndrome such as familiar adenomatous polyposis, Lynch Syndrome and so on
• Patients who had previous abdominal surgery
• Patients who are known to have colonic stricture or obstructing tumor from the results of radiography (X ray, CT scan or barium enema)
• Patients who are presenting acute surgical conditions such as severe colitis, megacolon and active gastrointestinal bleeding
• Patients who have inadequate bowel preparation
• Patients who reject to participate in this study