This prospective randomized controlled trial is to compare evaluate the optimal procedure sequence among patients undergoing water exchange colonoscopy.

Same-day esophagogastroduodenoscopy (EGD) and colonoscopy (bidirectional endoscopy; BDE) are commonly performed to evaluate for gastrointestinal conditions, such as active gastrointestinal bleeding, iron deficiency anemia, positive fecal occult blood tests and abdominal pain. EGD-colonoscopy sequence seems to be the reasonable choice for BDE using air or CO2. With the use of water exchange method, endoscopists have to remove the dirty fecal material and air bubble during insertion. Gas insufflated during EGD may get into the colon and make subsequent water exchange colonoscopy more difficult. The aim of this prospective randomized controlled trial is to compare evaluate the optimal procedure sequence among patients undergoing water exchange colonoscopy.

Inclusion Criteria:

• Patients underwent fully sedated bidirectional endoscopy in the physical check-up department

Exclusion Criteria:

• Patients who < 18 years old, > 80 years old
• Without standard colon preparation
• Allergy to meperidine or propofol
• With a history of partial colectomy
• Refusal to provide written informed consent
• American Society of Anesthesiology (ASA) risk Class 3 or higher