Abstract Background PCB is an important colonoscopy quality indicator that is recommended to be routinely collected by colorectal cancer screening programs and endoscopy quality improvement programs. Aims To create a standardized and reliable definition of PCB and set of rules for attributing the relatedness of PCB to a colonoscopy. Methods PCB events were identified from colonoscopies performed at the Forzani & MacPhail Colon Cancer Screening Centre. The Centre’s QI program reviews all emergency department visits and inpatient stays occuring within 30 days. Existing definitions and relatedness rules for PCB were reviewed by the authors and a draft definition and set of rules was created. Initial testing was performed using a set of 15 bleeding events. Information available for each event included the original endoscopy report and data abstracted from the emergency or inpatient record by a trained research assistant (CMM). The other six authors, all endoscopists, independently reviewed each event to determine if it met the definition of PCB and assessed its relatedness to the colonoscopy. The authors then met to review their ratings and revisions to the definition and rules were made. A validation set of 32 bleeding events were then reviewed to assess their interrater reliability by having three authors complete independent reviews and three authors complete a consensus review. The Kappa statistic was used to measure interrater reliability. Results PCB was defined as “Patient- or health care provider-reported rectal bleeding (other than blood on the toilet paper) and/or hemoglobin drop >2g within 30 days of procedure resulting in an emergency/urgent care center visit or hospital admission.” The relatedness criteria are shown in the table. The panel classified 28 of 32 events as meeting the definition of PCB and rated 7, 8 and 6 events as definitely, probably and possibly related to the colonoscopy, respectively. The Kappa for the definition of PCB for the three independent reviews was 0.82 (substantial agreement). The Kappa for the attribution of the PCB to the colonoscopy was 0.47 (moderate agreement). When reclassifying events as related or unrelated to the colonoscopy the agreement was higher (0.85). Conclusions A standardized definition of PCB and attribution rules achieved high interrater reliability and provides a template of required data for event adjudication by QI programs. Attribution Rules Relatedness Criteria Definite Active bleeding or adherent clot at 2nd colonoscopy Probable 3 of 1. ≤14 days 2. hot snare polypectomy 3. visible vessel at 2nd colonoscopy 4. HRP Possible 2 of probable criteria Unlikely 3 of 1. > 14 days 2. biopsy, cold snare or no polypectomy 3. not HRP 4. no high risk stigmata or no 2nd colonoscopy Unrelated Alternate source found HRP: High RIsk Polypectomy: size > 2 cm; 1–2 cm in right colon or pedunculated or antiplatelet/anticoagulant; immediate bleeding Funding Agencies None