Using data from a large commercial insurance claims database, this cohort study sought to identify factors associated with potentially missed appendicitis by comparing patients with a potentially missed diagnosis versus patients diagnosed with appendicitis on the same day in the emergency department. The researchers estimated the frequency of missed appendicitis was 6% among adults and 4.4% among children. Patients presenting with abdominal pain and constipation were more likely to have a missed diagnosis of appendicitis than patients presenting with isolated abdominal pain or abdominal pain with nausea and/or vomiting. Stratified analyses based on undifferentiated symptoms found that women and patients with comorbidities were more likely to have missed appendicitis.