Waterman 2013.

Study characteristics
Methods	Matched retrospective cohort study. Study period: 2000 to 2010
Participants	Country: Canada. CD and UC patients who underwent abdominal surgery.
Interventions	1. Preoperative infliximab (n= 195) within 180 days of surgery 2. No preoperative infliximab (n= 278)
Outcomes	Wound infection, anastomotic leak, intraabdominal abscess, extraabdominal infections within 30 days of surgery
Notes	NOS low risk of bias overall
Risk of bias
Bias	Authors' judgement	Support for judgement
Representativeness of the exposed cohort	Low risk	CD and UC patients who underwent abdominal surgery from 2000 to 2010
Selection of the non exposed cohort	Low risk	Both groups obtained from the same database and time period.
Ascertainment of exposure	Low risk	Patient charts 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)	Low risk	Groups matched for steroids
Comparability of cohorts (Controlled for additional factor)	Low risk	Groups also matched for surgical procedure, IBD subtype and patient age
Assessment of outcome	Low risk	Patient charts reviewed
Was follow‐up long enough for outcomes to occur	Low risk	30 days
Adequacy of follow up of cohorts	Low risk	Patients without adequate clinical records documenting 30 day outcomes were excluded