The Influence of Body Mass Index on the Accuracy of Ultrasound and Computed Tomography in Diagnosing Appendicitis in Children

Background: Appendicitis is a common pediatric surgical diagnosis that is often guided by radiographic imaging such as ultrasound (US) or computed tomography (CT). Whereas US is preferred because it avoids radiation, data suggest it is less sensitive than CT in diagnosing appendicitis. Body habitus, characterized by body mass index (BMI), may affect the diagnostic value of advanced radiological imaging in children with abdominal pain. Objective: The aim of the study was to determine the relationship between BMI and accuracy of US and CT scan for suspected appendicitis. Methods: We conducted a prospective, observational study of children with suspected appendicitis who presented to the emergency department and underwent radiographic evaluation from June 2007 to July 2008. Patient height and weight were obtained to calculate BMI and correlating z score, zBMI. Patients were grouped into standardized categories: underweight/normal weight, overweight, and obese based on zBMI. Emergency physicians ordered imaging as guided by standard clinical management. Pediatric attending radiologists evaluated all imaging studies; final radiology reports were reviewed and coded by 2 independent reviewers. Appendicitis was defined by pathology. Statistical analysis was performed to evaluate test performance of US and CT in diagnosing appendicitis as related to BMI. Results: Over the study period, 176 patients with suspected appendicitis underwent US and/or CT. Mean age was 11.8 +/- 4.2 years; 42% were male. zBMI ranged from -2.78 to 2.75 (mean, 0.59); 70 children (40%) were overweight or obese; 73 (42%) had appendicitis. Ultrasound was performed on 147 (84%), and CT on 128 children (73%); 99 children (56%) had both studies. The overall sensitivity for US in diagnosing appendicitis was 38% (95% confidence interval [CI], 26%-52%) with a specificity of 97% (95% CI, 90%-99%). In the underweight/normal weight group, the sensitivity of US was 45% (95% CI, 27%-64%); in the overweight group, 35% (95% CI, 15%-61%), and in the obese group, 22% (95% CI, 4%-60%). The sensitivity and specificity of CT were 96% (95% CI, 86%-99%) and 97% (95% CI, 90%-100%) and did not vary by zBMI class. Conclusions: The sensitivity and specificity of CT for appendicitis are excellent regardless of BMI. Despite a trend of decreasing sensitivity with increasing BMI when using US, no statistically significant difference was found, likely because of type II error.