Table 2.

Characteristics of included studies reporting on the association between smoking and colectomy in ulcerative colitis

Study	Country	Source of Patients	Definition of Smoking	Timing of Smoking	Duration of Follow-up	Sample Size	Adjusted Estimates
Beaugerie 2001 [15]	France	Tertiary care	Current: >7 cigarettes/week for ≥6 months after diagnosis Former: Quit for >12 months at time of evaluation	At colectomy or study enrolment	Mean: Current smokers: 10 years Former smokers: 16 years Never smokers: 14 years	96a	Crudeb
Boyko 1988 [16]	USA	HMO	Current: Smoked >100 cigarettes and continued to smoke or initiated smoking after diagnosis Former: Smoked >100 cigarettes and quit prior to diagnosis and did not restart Never: Smoked <100 cigarettes	At diagnosis	Median (range): 8 years (1, 35)	206	Age; sex
Frolkis 2016 [14]	UK	THIN database	Patient coded in EMR as Current, Former, or Never Smoker within one year of index (diagnosis date)	At diagnosis	Median (Q1, Q3): 5 years (3, 8)	3600	Age at diagnosis; sex; use of immunosuppressants; steroid use within 90 days of diagnosis
Hoie 2007 [17]	Europec	Population-basedd	Current: Maintained same smoking behaviour throughout follow-up Former: Dates when patient started and stopped smoking were compared with year of disease onset and colectomy Never: Patients who had never been daily cigarette smokerse	At diagnosis	Median (range): 10 (9, 12) years	771	Crude

HMO health maintenance organization

^a32 former smokers were matched with 32 current smokers and 32 never smokers

^bCrude hazard ratios were estimated from Kaplan-Meier curves using the method proposed by Guyot et al.[25]

^cGreece, Israel, Italy, Spain, Denmark, the Netherlands, Norway

^dAdditional information on study methodology obtained from Shivananda et al.[32]

^eStudy also reports a hazard ratio for patients with unknown smoking status relative to individuals who never smoked