Diagnostic accuracy of laboratory and ultrasound findings in patients with a non-visualized appendix

Ultrasound (US) and laboratory testing are initial diagnostic tests for acute appendicitis. A diagnostic dilemma develops when the appendix is not visualized on US. Objective: To determine if specific US findings and/or laboratory results predict acute appendicitis when the appendix is not visualized. Methods: A prospective study was conducted on children (birth-18 yrs) presenting to the pediatric emergency department with suspected acute appendicitis who underwent right lower quadrant US. Children with previous appendectomy, US at another facility, or eloped were excluded. US findings analyzed: inflammatory changes, right lower quadrant and lower abdominal fluid, tenderness during US exam and lymph nodes. Diagnoses were confirmed via surgical pathology. Results 1252 subjects were enrolled, 60.8% (762) had appendix visualized and 39.1% (490) did not. In children where the appendix was not seen, 6.7% [33] were diagnosed with appendicitis. Among patients with a non-visualized appendix, the likelihood of appendicitis was significantly greater if: inflammatory changes in the RLQ (OR 18.0, 95% CI 4.5-72.1), CRP >0.5 mg/dL (OR 2.64, 95% CI 1.0-6.8), or WBC > 10 (OR 4.36, 95% CI 1.66-11.58). Duration of abdominal pain >3 days was significantly less likely associated with appendicitis in this model (OR 0.34, 95% CI 0.003-0.395). Combined, the absence inflammatory changes, CRP < 0.5 mg/dL, WBC < 10, and pain, ≤3 days had a NPV of 94.0%. Conclusion When the appendix is not visualized on US, predictors for appendicitis include the presence of inflammatory changes in the RLQ, an elevated WBC/CRP and abdominal pain <3 days.