AbstractSTUDY QUESTIONWhat is the additional value of the comprehensive complication index (CCI) and ClassIntra system (classification for intraoperative adverse events (ioAEs)) in adverse event (AE) reporting in (deep) endometriosis (DE) surgery compared to only using the Clavien–Dindo (CD) system?SUMMARY ANSWERThe CCI and ClassIntra are useful additional tools alongside the CD system for a complete and uniform overview of the total AE burden in patients with extensive surgery (such as DE), and with this uniform data registration, it is possible to provide greater insight into the quality of care.WHAT IS KNOWN ALREADYUniform comparison of AEs reported in the literature is hampered by scattered registration. In endometriosis surgery, the usage of the CD complication system and the CCI is internationally recommended; however, the CCI is not routinely adapted in endometriosis care and research. Furthermore, a recommendation for ioAEs registration in endometriosis surgery is lacking, although this is vital information in surgical quality assessments.STUDY DESIGN, SIZE, DURATIONA prospective mono-center study was conducted with 870 surgical DE cases from a non-university DE expertise center between February 2019 and December 2021.PARTICIPANTS/MATERIALS, SETTING, METHODSEndometriosis cases were collected with the EQUSUM system, a publicly available web-based application for registration of surgical procedures for endometriosis. Postoperative adverse events (poAEs) were classified with the CD complication system and CCI. Differences in reporting and classifying AEs between the CCI and the CD were assessed. ioAEs were assessed with the ClassIntra. The primary outcome measure was to assess the additional value toward the CD classification with the introduction of the CCI and ClassIntra. In addition, we report a benchmark for the CCI in DE surgery.MAIN RESULTS AND THE ROLE OF CHANCEA total of 870 DE procedures were registered, of which 145 procedures with one or more poAEs, resulting in a poAE rate of 16.7% (145/870), of which in 36 cases (4.1%), the poAE was classified as severe (≥Grade 3b). The median CCI (interquartile range) of patients with poAEs was 20.9 (20.9–31.7) and 33.7 (33.7–39.7) in the group of patients with severe poAEs. In 20 patients (13.8%), the CCI was higher than the CD because of multiple poAEs. There were 11 ioAEs reported (11/870, 1.3%) in all procedures, mostly minor and directly repaired serosa injuries.LIMITATIONS, REASONS FOR CAUTIONThis study was conducted at a single center; thus, trends in AE rates and type of AEs could differ from other centers. Furthermore, no conclusion could be drawn on ioAEs in relation to the postoperative course because the power of this database is not robust enough for that purpose.WIDER IMPLICATIONS OF THE FINDINGSFrom our data, we would advise to use the Clavien–Dindo classification system together with the CCI and ClassIntra for a complete overview of AE registration. The CCI appeared to provide a more complete overview of the total burden of poAEs compared to only reporting the most severe poAEs (as with CD). If the use of the CD, CCI, and ClassIntra is widely adapted, uniform data comparison will be possible at (inter)national level, providing better insight into the quality of care. Our data could be used as a first benchmark for other DE centers to optimize information provision in the shared decision-making process.STUDY FUNDING/COMPETING INTEREST(S)No funding was received for this study. The authors have no conflicts of interest to declare.TRIAL REGISTRATION NUMBERN/A.