Ultrasound evaluation of appendicitis: importance of the 3 × 2 table for outcome reporting

Despite a relatively high frequency of appendix nonvisualization when using ultrasound to diagnose appendicitis, many studies either fail to report these results or inconsistently analyze outcomes.

The objective of this study is to determine the most transparent and accurate way of reporting and analyzing ultrasound results for the diagnosis of appendicitis.

This was an observational cohort study of emergency department patients age 18 years or older who underwent right lower quadrant ultrasonography from September 2010 to October 2011. Patient characteristics, imaging, pathology, and follow-up data were analyzed. Test characteristics were calculated using conventional 2 × 2 contingency table analysis excluding inconclusive ultrasound results and an intention-to-diagnose approach with a 3 × 2 table.

Sixty-five patients were included. Forty-four (68%) patients had a nonvisualized appendix resulting in an overall diagnostic yield of 32%. Twenty-one patients had a visualized appendix (14 [22%] negative and 7 [11%] positive for appendicitis). Using 2 × 2 contingency table analysis, sensitivity and specificity were 100%. Using the 3 × 2 table with and the intention-to-diagnose principle, sensitivity was 70% and specificity was 25%. Three (7%) of 44 patients with a nonvisualized appendix had appendicitis (likelihood ratio = 0.40).

We suggest reporting ultrasound results using a 3 × 2 table (including nonvisualized findings) but using the traditional 2 × 2 type of analysis for test characteristic calculations. This approach allows for the determination of diagnostic yield and calculation of likelihood ratios when the appendix is not visualized. This approach to reporting should be considered for all types of diagnostic ultrasound studies.