Abstract:Objective To systematically review the effect of colonoscopy timing on the outcomes of patients with acute lower gastrointestinal bleeding (LGIB).Methods PubMed, Medline, Embase, The Cochrane Library, VIP, CNKI and WanFang Data were electronically searched to collect studies on the effect of colonoscopy timing on the outcomes of patients with acute LGIB from inception to September, 2022. Two reviewers independently screened the literature, extracted data, and evaluated the risk of bias of the included studies. Subsequently, Meta-analysis was performed using RevMan 5.4 software on rebleeding rate, endoscopic hemostasis rate, bleeding source localization rate, overall mortality rate, etc.Results 12 retrospective studies and 4 randomized controlled trials (RCT) involving 400 180 patients were included. The results of retrospective studies showed that compared with elective colonoscopy, urgent colonoscopy was superior in the following aspects: endoscopic hemostasis rate (OR^ = 1.64, 95%CI: 1.07 ~ 2.52, P = 0.020), length of stay (MD = -1.67, 95%CI: -2.92 ~ -0.42, P = 0.009), surgery rate (OR^ = 0.52, 95%CI: 0.42 ~ 0.64, P = 0.000), transfusion rate (OR^ = 0.83, 95%CI: 0.78 ~ 0.88, P = 0.000). While no significant differences were observed in rebleeding rate, overall mortality rate, embolization rate and bleeding source localization rate between the two groups. The results of RCT showed that urgent colonoscopy was superior to elective group only on endoscopic hemostasis rate (OR^ = 1.74, 95%CI: 1.04 ~ 2.93, P = 0.040) and bleeding source localization rate (OR^ = 2.31, 95%CI: 1.28 ~ 4.17, P = 0.006), there was no statistically significant difference in other outcome indicators between the two groups (P > 0.05).Conclusion Compared with elective colonoscopy, urgent colonoscopy can’t reduce the rate of rebleeding and overall mortality, but may improve the success rate of endoscopic hemostasis and meanwhile reduce the length of stay, rate of surgery and transfusion. Clinicians should make decisions according to the actual situation.