Study Sample
The present cohort study consisted of case and comparison groups. People who underwent appendectomy (International Classification of Diseases, Ninth Revision, Clinical Modification [ICD-9-CM ] codes 47.0 and 47.1) were recruited as the case group.
We defined the first operation date of appendectomy as the index date. Patients with IBD diagnosis (ICD-9-CM codes 555 and 556) before the index date were excluded. The comparison group consisted of people without a history of IBDs and appendectomy. Participants aged <20 years were excluded from both the groups. Controls were matched with patients in the case group based on sex, age, and index year with a 1:1 ratio. All participants were followed from the index date to withdrawal from the NHIP, death, or end of the study (December 31, 2013).