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).