Background: Same-day bidirectional endoscopy is commonly performed in clinical practice. However, the optimal sequence of procedures for same-day bidirectional endoscopy has not been established. The purpose of this study was to compare colonoscopy performance and quality between patients who underwent colonoscopy before or after esophagogastroduodenoscopy (EGD). Methods: A total of 1,103 patients were prospectively randomized into either the EGD-colonoscopy or colonoscopy-EGD sequence groups. Three patients who had incomplete cecal intubation due to structural obstruction were excluded from the analysis. During colonoscopy, colonoscopic parameters including difficult cecal intubation (cecal intubation failure and prolonged insertion), insertion time, and adenoma detection rate were measured. Out of 1,100 patients, 524 patients without sedation completed a questionnaire designed to assess subjective discomfort experienced. Results: The colonoscopy completion rate was 99.5 %, and the rate of difficult cecal intubation was 14.5 %. The time from insertion to reaching the cecum (minutes:seconds, 06:32 ± 04:26 vs. 06:40 ± 04:09, p = 0.649), difficult cecal intubation ratio (76 of 550 vs. 83 of 550, p = 0.593), and colonoscopic adenoma detection rate (29.8 vs. 25.5 %, p = 0.106) did not differ between the groups. On multivariate analysis, difficulty with cecal intubation increased specifically in women, in patients aged 55 years and over, in patients with poor bowel preparation, and in patients who had undergone previous abdominal surgery. Subjective discomfort after EGD was higher in the colonoscopy-EGD sequence group. Conclusions: The procedural sequence did not affect colonoscopy performance and quality in same-day bidirectional endoscopy, and factors such as old age, female gender, poor bowel preparation, and previous abdominal surgery were confirmed to adversely affect colonoscopy. In addition, the EGD-colonoscopy sequence induced less subjective discomfort during EGD.
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