Acute appendicitis is one of the most common reasons for acute abdomen. Current epidemiological and clinical studies clearly suggest that appendicitis consists of a diverse spectrum of variable disease severity forms. This results in multiple treatment alternatives as uncomplicated acute appendicitis can be treated with antibiotics and even with only symptomatic treatment, while complicated appendicitis mainly requires emergency operation. Appendicitis can be broadly divided into uncomplicated and complicated appendicitis often distinguished using CT imaging. The knowledge gap is the accurate differential diagnosis with uniform definitions of appendicitis severity especially in the middle ground ”grey area” between these two disease forms. Accurate differential diagnosis allows optimization of all treatment options for both patients and healthcare systems.

The main aim of this thesis was to assess the differences in blood-culture positivity rate between uncomplicated and complicated acute appendicitis and to improve the accuracy of CT imaging in differential diagnostics between the different disease severities. In addition, the longer-term outcomes of antibiotic and symptomatic treatment for uncomplicated acute appendicitis were assessed. Study I investigated blood culture positivity in patients with acute appendicitis. Studies II and III evaluated CT imaging findings to better identify and rule out complicated acute appendicitis using a large prospective patient cohort. Study II assessed the association of appendicolith CT characteristics with appendicitis severity and study III aimed to identify potential prognostic CT factors for complicated appendicitis. Study IV was a 3-year follow-up of the APPAC III randomized double-blind clinical pilot trial comparing antibiotics to placebo in adults with CT-confirmed uncomplicated appendicitis.

Complicated appendicitis was associated with higher rates of bacteremia compared to uncomplicated appendicitis, and the overall prevalence of blood culture positivity in patients with acute appendicitis was 9%. The presence of appendicolith in patients with acute appendicitis increased the risk of complicated appendicitis. CT findings associated with complicated acute appendicitis included larger appendiceal diameter, appendiceal wall enhancement defect and accumulation of fluid around the appendix. At 3 years, there was no difference between antibiotics and placebo in terms of treatment success with an overall appendectomy rate of 35%.