Yu 2019.

Study characteristics
Methods	Retrospective cohort. Study period 2006 to 2015
Participants	Country: Korea. Patients with Crohn's disease who underwent bowel resection with or without anastomosis or strictureplasty. Patients excluded if they had open biopsy or stoma formation/reversal only
Interventions	1. Preoperative steroids (n= 39) 2. Preoperative immunomodulators (azathioprine, 6MP, methotrexate) (n= 211) 3. Preoperative anti‐TNF medications within 8 weeks of surgery (n= 32)
Outcomes	30 day infectious and non‐infectious postoperative complications. Infectious complications include intra‐abdominal sepsis, enterocutaneous fistula, wound infection and extra‐abdominal infection
Notes	NOS very high risk of bias overall
Risk of bias
Bias	Authors' judgement	Support for judgement
Representativeness of the exposed cohort	Low risk	Patients with Crohn's disease who underwent bowel resection with or without anastomosis or strictureplasty.
Selection of the non exposed cohort	Low risk	Both groups obtained from the same hospital and time period
Ascertainment of exposure	Low risk	Medical records reviewed
Demonstration that outcome of interest was not present at start of study	Unclear risk	Information not provided
Comparability of cohorts (Controlled for critical factor/other medications)	High risk	Univariate analysis
Comparability of cohorts (Controlled for additional factor)	High risk	Univariate analysis
Assessment of outcome	Low risk	Medical records reviewed
Was follow‐up long enough for outcomes to occur	Low risk	30 days
Adequacy of follow up of cohorts	Unclear risk	Information not provided