Water exchange colonoscopy was performed as described previously (Am. J. Gastroenterol. 2017 04;112(4)). Patients were randomly allocated to WE group or WE+AA group. In WE+AA group, water containing 1% of AA was used during insertion of colonoscopy instead of water alone. A bottle of 20ml pure AA was placed abutting the water pump, which make it possible to keep patients blinded to group allocation. Endoscopist using a high-definition wide-angle Olympus or Fujinum colonoscope performed all of the colonoscopies, with a withdrawal time of at least 6 min. The observation was initially performed without the assistance of cap or the use of magnification or electronic staining (NBI, BLI, LCI or AFI et al.). Scopolamine butylbromide or glucogon was not routinely administrated.