Sonography in the evaluation of acute appendicitis: are negative sonographic findings good enough?

The purpose of this study was to evaluate the negative predictive value (NPV) of sonography in the diagnosis of acute appendicitis.

Right lower quadrant sonograms of 193 patients (158 female and 35 male; age range, 3-20 years) with suspected acute appendicitis over a 1-year period were retrospectively reviewed. Sonographic findings were graded on a 5-point scale, ranging from a normal appendix identified (grade 1) to frankly acute appendicitis (grade 5). Sonographic findings were compared with subsequent computed tomographic (CT), surgical, and pathologic findings. The diagnostic accuracy of sonography was assessed considering surgical findings and clinical follow-up as reference standards.

Forty-nine patients (25.4%) had appendicitis on sonography, and 144 (74.6%) had negative sonographic findings. Computed tomographic scans were obtained in 51 patients (26.4%) within 4 days after sonography. These included 39 patients with negative and 12 with positive sonographic findings. Computed tomography changed the sonographic diagnosis in 10 patients: from negative to positive in 3 cases and positive to negative in 7. Forty-three patients (22.2%) underwent surgery. The surgical findings were positive for appendicitis in 37 (86%) of the 43 patients who had surgery. Patients with negative sonographic findings who, to our knowledge, did not have subsequent CT scans or surgery were considered to have negative findings for appendicitis. Seven patients with negative sonographic findings underwent surgery and had appendicitis; therefore, 137 of 144 patients with negative sonographic findings did not have appendicitis. On the basis of these numbers, the NPV was 95.1%.

Sonography has a high NPV and should be considered as a reasonable screening tool in the evaluation of acute appendicitis. Further imaging could be performed if clinical signs and symptoms worsen.