Aim: To define benchmark outcomes in minimally-invasive primary bariatric surgery.

Design: Multicenter retrospective cohort study.

Assessed outcomes: Morbidity as defined by the Clavien-Dindo classification for surgical complications, the Comprehensive Complication Index® (CCI®) at discharge, at 3 months and at latest follow-up. Evolution of body mass index (BMI) will be also analyzed.

Hospital eligibility: High volume centers (> 200 bariatric operations per year) from at least three continents, maintaining a prospective database, as well as having published previously critically on their outcome.

Study population: Adult patients who underwent primary minimally invasive (laparoscopic / robotic) Roux-en-Y gastric bypass or sleeve gastrectomy from 1st of June 2012 to 31st of May 2017.

Patient Exclusion criteria: detailed later.

Data collection Deadline: 1st September 2017 - 30 April 2018

Background With the growing complexity and cost of modern surgical practice, quality assessment becomes mandatory. The notion of quality and quality assessment is widely recognized and used in the world of business and manufacturing. A possible tool of quality assessment is benchmarking. Benchmarking is a process of measuring performance by comparison to the outcomes achieved by the best "service provider" in a specific domain. Usually, a benchmark describes the ''best possible'' outcome of a benchmarking subject to whom comparison can be performed. In the surgical community, however, such benchmarks - best possible outcomes - for specific procedures, not just the pooled overall performance, are lacking.

In 2016, a first landmark study defining benchmark outcomes for liver resection was published in Annals of Surgery by a group of international authors invited and guided by our department. More recently, further surgical outcomes (liver transplantation, minimally invasive esophagectomy) have been benchmarked and have been accepted for publication.

Since laparoscopic bariatric surgery has become a standardized and widely performed procedure worldwide, quality assessment is of major importance. To identify the best possible outcomes (i.e. the benchmarks), data from high-volume centers (based on official IFSO criteria) in low risk patients will be analyzed. These benchmarks will serve as "optimal outcomes" for comparison with single center outcomes, high-risk patients and future developments.

Aim The primary aim is to define benchmark outcomes based on assessment of post procedural complications according to the Clavien-Dindo classification for surgical complications and the comprehensive complication index CCI™ at discharge and at 90-days. The CCI® expresses morbidity on a continuous numeric scale from 0 (no complications) to 100 (death) by weighing all postoperative complications according to the Clavien-Dindo classification for their respective severity. Secondary outcome measure are patient survival and excess BMI loss (EBMIL).

Data Security This multicenter international study is designed to harvest prospectively collected retrospective data via an encrypted (i.e. Secure Sockets Layer (SSL) protocol) online platform (https://bbenchmarks.org/) that meets Food and Drug Administration (FDA) standards and is accessible only by secured login membership.

Confidential center specific data: Centers' outcomes will be individually analyzed in a first step to screen for center-specific differences. Benchmarks will be computed from each center's results in a second step. No center-specific data will be published. Instead, all complications or adverse outcomes will be anonymously reported, as fractions of the total study population. Each center, of course, will be free to publish their own data, as they wish.

Further use of cohort data: Future studies based on the collected data may emerge from this multicenter study, such as comparing outcomes in patients with or without specific comorbidities with benchmark outcomes. For further data usage, additional ethics approval may be required.

Inclusion Criteria:

• Adult patients of 18-65 years
• Low risk profile (please read "exclusion criteria"),
• Maximum preoperative BMI of 50 kg/m2
• Primary laparoscopic/robotic proximal Roux-en-Y gastric bypass or sleeve gastrectomy
• Documented follow-up of at least 90 days

Exclusion Criteria:

• Open surgery
• Previous intra-abdominal surgery (including previous bariatric surgery)
• Pre-operative BMI over 50 kg/m2
• Age over 65 years
• Cardiovascular disease (e.g. cardiac arrhythmia, stroke, coronary artery disease) (Hypertension is allowed)
• History of thromboembolic events and/or therapeutic anticoagulation
• Diabetes mellitus (Type I and Type II, as defined by the American Diabetes Association)
• Obstructive sleep apnea (recurrent episodes of upper airway collapse during sleep)
• Chronic obstructive pulmonary disease (FEV1/FVC<0.7)
• Chronic kidney disease (eGFR < 30ml/min/1.72 m2)
• Inflammatory bowel disease (ulcerative colitis, Crohn's)
• Immunosuppression therapy (e.g. steroids, calcineurin inhibitors, etc)
• Patients who underwent associated procedures (for example: cholecystectomy, hiatoplasty, liver biopsy)
• ASA score > 2

Adult patients with severe obesity with a lower risk profile who undergo bariatric surgery

the study is based on de-anonymised data, so it is impossible to backtrack it to a patient. only overall data will be shared, not individual data