Patient Groups
Primary population proxy : appendicitis group. Patients with a primary appendicitis-related diagnosis were included in the appendicitis group if they had the following characteristics:
1) a primary diagnosis of an appendicitis-related condition (ICD-9 codes 540–542) at index admission;
2) no prior or concurrentindication (in relation to the index appendicitis admission) of any schizophrenia spectrum disorder (codes 295.x, schizophrenic disorders; 297.x, delusional disorders; 298.x, other nonorganic psychoses; or 301.2, schizoid personality disorder);
3) no concurrent diagnoses at index admission of drug-induced psychoses(code 292.1, drug-induced psychotic disorders); and
4) no prior,concurrent, or subsequent indication of any alcohol or drug use diagnosis. The codes used are listed in Table 1.