Independent Predictors of Acute Appendicitis on CT with Pathologic Correlation 1 Elizabeth P. Ives, MD, Susan Sung, MD, Peter McCue, MD, Haroon Durrani, MD, Ethan J. Halpern, MD Rationale and Objectives. To assess computed tomographic (CT) signs that have been described in published studies for the diagnosis of appendicitis to identify independent findings that predict appendicitis. Methods and Materials. A retrospective database search identified 67 patients with a CT scan of the abdomen/pelvis and pathologic evaluation of the appendix, including 41 with appendicitis and 26 with a normal appendix on pathologic exam- ination. Each computed tomogram was re-evaluated by three independent, blinded observers who evaluated appendix di- ameter, enhancement of the appendix, thickening of the appendix, presence of an appendicolith, infiltration of peri-appen- diceal fat, focal cecal thickening, local lymphadenopathy, fluid collections, non-appendiceal bowel thickening, non-periap- pendiceal infiltration of fat, and comparison of peri-appendiceal fat infiltration to thickening of adjacent bowel loops. Results. Mean diameter of the normal appendix (6.7  2.2 mm) was significantly lower than that of the inflamed appen- dix (12.1  4.3 mm; P  .001). Significant univariate predictors of appendicitis included appendix diameter 8 mm (odds ratio [OR] 34.8), enhancement of the appendix (OR 4.4), thickening of the appendix (OR 4.3), infiltration of peri- appendiceal fat (OR 5.5), focal cecal thickening (OR 5.1), non-appendiceal bowel thickening (OR 0.4), and non-periap- pendiceal infiltration of fat (OR = 0.3). Of these variables, only appendix diameter and enhancement of the appendix were significant independent predictors of appendicitis on multivariate analysis. An overall diagnostic impression based on all secondary signs was less accurate than a diagnosis based on appendix diameter alone (receiver-operating characteristic analysis: Az = 0.80 vs. Az = 0.91, P = .02). Sensitivity/specificity of appendix diameter was 84%/87% using a cutoff between 8 and 9 mm and 97%/48% using a cutoff between 6 and 7 mm. Conclusion. Appendix diameter is the best single diagnostic criterion for appendicitis on CT scan. A cutoff between 8 and 9 mm provided the best balance of sensitivity/specificity in our study population, whereas a cutoff between 6 and 7 mm improved sensitivity at the expense of specificity. The presence of appendiceal enhancement provided additional diagnos- tic information, but other secondary signs of appendicitis did not improve diagnostic accuracy. Key Words: Appendix; appendicitis; computed tomography; pathology. © AUR, 2008 Appendicitis is the most commonly encountered cause of abdominal pain requiring surgical management (1). De- layed or inaccurate diagnosis of appendicitis can lead to significant morbidity and mortality. Within the United States, there are a reported one million hospital days per year from 250,000 cases annually of appendicitis (2). The incidence of appendicitis is greatest in the second decade of life (3). Computed tomography has been advocated as a cost- efficient and accurate way to diagnose appendicitis, par- ticularly in the setting of an inconclusive clinical exami- nation (4–8). In many hospitals, computed tomography is considered the standard of care for the evaluation of sus- pected appendicitis (7). Computed tomographic (CT) find- ings described in published studies that suggest a diagno- Acad Radiol 2008; 15:996 –1003 1 From the Departments of Radiology (E.P.I., S.S., H.D., E.J.H.) and Pathol- ogy, 132 S. 10th St., 10th Floor, Thomas Jefferson University, Philadelphia, PA 19107 (P.M.). Received November 21, 2007; accepted February 8, 2008. Address correspondence to: E.P.I. e-mail: epives@yahoo.com © AUR, 2008 doi:10.1016/j.acra.2008.02.009 996