Background. Acute appendicitis (“appendicitis”) is one of the most common abdominal surgical emergencies worldwide. In spite of this, the diagnostic pathways are highly variable across countries, between centres and physicians. This has implications for the use of resources, exposure of patients to ionising radiation and patient outcome. The aim of this thesis is to construct and validate a diagnostic appendicitis score, to evaluate new inflammatory markers for inclusion in the score, and explore the effect of implementing a structured management algorithm for patients with suspected appendicitis. Also, we compare the outcome of management with routine diagnostic imaging versus observation and selective imaging in equivocal cases.

Methods. In study I, the Appendicitis Inflammatory Response (AIR) score was constructed from eight variables with independent diagnostic value (right lower quadrant pain, rebound tenderness or muscular defence, WBC count, proportion of polymorphonuclear granulocytes, CRP, body temperature and vomiting). Its diagnostic properties were evaluated and compared with the Alvarado score. In study II, we performed an external validation and evaluation of novel inflammatory markers for inclusion in the score on patients with suspected appendicitis at two Swedish hospitals. In study III we externally validated and evaluated the impact of an AIR-scorebased algorithm assigning patients to a low or high risk of having appendicitis in an interventional multicentre study involving 25 Swedish hospitals and 3791 patients. In study IV, we compared the efficiency of routine diagnostic imaging with repeated clinical assessment followed by selective imaging in a randomised trial of 1028 patients with equivocal signs of appendicitis, as indicated by an intermediate AIR score, from study III.

Main results. In study I we found that the AIR score could assign 63% of the patients to either a high- or low-risk group of appendicitis with an accuracy of 97%, which compared favourably with the Alvarado score. In study II, the diagnostic properties of the AIR score proved to be  reproducible, but the inclusion of novel inflammatory markers did not improve the diagnostic accuracy. In study III, the AIR-score-based algorithm led to a reduction in negative explorations, operations for nonperforated appendicitis and hospital admissions in the low-risk group and reduced use of imaging in both low- and high-risk groups. In study IV, routine imaging led to more operations for nonperforated appendicitis but had no effect on negative explorations or perforated appendicitis.

Conclusions. The AIR score was found to have promising diagnostic properties that were not improved further with the inclusion of novel inflammatory variables. Structured management of patients with suspected appendicitis according to an AIR-score-based algorithm may improve outcome while reducing hospital admissions and use of imaging. Patients with equivocal signs of appendicitis do not benefit from routine imaging which may lead to an increased detection of, and treatment for, uncomplicated cases of appendicitis that are otherwise allowed to resolve spontaneously.