Aberra 2003.

Study characteristics
Methods	Retrospective cohort. Study period: 1992 to 2000
Participants	Country: USA. Patients with UC or CD who underwent elective bowel surgery. Patients treated with infliximab, mycophenolate, or tacrolimus were excluded
Interventions	1. Azathioprine/6‐mercaptopurine within 2 weeks of surgery (n=52) 2. Preoperative corticosteroids (n=90) 3. No preoperative immunosuppressants or corticosteroids (n=51)
Outcomes	Wound infection, sepsis, p n e umonia, peritonitis, abdominal abscess and wound dehiscence within 30 days of surgery
Notes	NOS low risk of bias overall. Adjusted O Rs for corticosteroids and azathioprine/6‐mercaptopurine were obtained from multivariate regression model.
Risk of bias
Bias	Authors' judgement	Support for judgement
Representativeness of the exposed cohort	Low risk	All UC and CD patients undergoing elective bowel surgery at University of Pennsylvania Health system from 1992 to 2000
Selection of the non exposed cohort	Low risk	Both groups obtained from the same hospital and time period
Ascertainment of exposure	Low risk	All charts were manually searched to determine medication exposure
Demonstration that outcome of interest was not present at start of study	Unclear risk	No information was provided
Comparability of cohorts (Controlled for critical factor/other medications)	Low risk	Azathioprine/6‐mercaptopurine analysis controlled for corticosteroids and vice versa
Comparability of cohorts (Controlled for additional factor)	Low risk	Also controlled for CD, disease refractory to medication, age >38 years, and surgery duration >241 minutes
Assessment of outcome	Low risk	Inpatient medical records were examined
Was follow‐up long enough for outcomes to occur	Low risk	Patients were followed until discharge from hospital (median 8 days, range 1‐37 days)
Adequacy of follow up of cohorts	Unclear risk	No information was provided