Incidence of acute appendicitis in patients with equivocal CT findings.

OBJECTIVE: The purpose of our study was to determine the incidence of acute appendicitis in patients for whom the CT interpretation is deemed equivocal.
MATERIALS AND METHODS: Of 1,344 patients referred for CT with suspected appendicitis between January 1998 and December 2002, 172 patients were identified in whom the radiographic findings were equivocal. Two radiologists reviewed the equivocal CT cases, reassessing appendiceal size and the presence of right lower quadrant stranding, fluid, or an appendicolith. The reviewers' findings were correlated with surgical pathology reports and clinical follow-up.
RESULTS: Fifty-three (31%) of 172 patients with indeterminate findings on CT scans were subsequently diagnosed with appendicitis. For reviewers 1 and 2, respectively, appendicitis was present in five (14%) of 36 and six (13%) of 47 patients who had isolated appendiceal diameter less than 9 mm, and in 11 (52%) of 21 and in 10 (50%) of 20 patients who had isolated appendiceal dilatation equal to or greater than 9 mm. If a normal diameter appendix (< 6 mm) was visualized in a patient who had right lower quadrant stranding or fluid, appendicitis was present in only one (17%) of six and in four (27%) of 15 patients for reviewers 1 and 2, respectively. If the appendix could not be identified but there was right lower quadrant stranding or fluid, appendicitis was present in seven (37%) of 19 and in eight (53%) of 15 patients.
CONCLUSION: Appendicitis is encountered in about 30% of patients with equivocal findings on CT, and the diagnosis should be considered in most of these patients if they are appropriately symptomatic. However, when the appendix measures less than 9 mm alone, the likelihood of appendicitis is much smaller.