What is the most appropriate diagnostic test for detecting acute appendicitis?

Diagnostic Approach to Acute Appendicitis

CT scan with intravenous contrast is the most appropriate diagnostic test for detecting acute appendicitis in adults due to its high sensitivity of 96% and specificity of 93%. 1

Diagnostic Algorithm for Suspected Appendicitis

Initial Risk Stratification

• Based on clinical presentation, patients can be categorized into low, intermediate, and high risk groups
• For intermediate to high-risk patients, imaging is warranted to confirm diagnosis

Optimal Imaging Approach

Adults:

1. First-line: CT abdomen and pelvis with IV contrast

   • Sensitivity: 96% (95% CI: 92-98%)
   • Specificity: 93% (95% CI: 90-95%)
   • Positive predictive value: 92% following a positive result
   • Negative predictive value: 96% following a negative result 1, 2

2. CT technique considerations:

   • IV contrast significantly improves diagnostic accuracy compared to non-contrast CT
   • IV contrast enhances sensitivity (96% vs 91% for non-contrast) 2
   • Allows better characterization of complications such as perforation and abscess 3
   • Focused CT from L2/L3 to pubic symphysis is sufficient in most cases 3

Children:

1. First-line: Ultrasound (RLQ or complete abdomen)

   • Reduces radiation exposure
   • If equivocal or non-diagnostic, proceed to CT or MRI 3

2. Second-line (if ultrasound is equivocal):

   • CT with IV contrast (sensitivity 91%, specificity 98%) 3
   • MRI (sensitivity 94%, specificity 94% with contrast) 3

Advantages of CT with IV Contrast

• Highest diagnostic accuracy among all imaging modalities
• Better visualization of the appendix compared to non-contrast CT 4
• Ability to identify alternative diagnoses when appendicitis is not present
• Reduced negative appendectomy rates (1.7-7.7% with preoperative CT vs 16.7% with clinical evaluation alone) 1
• Characterization of complications (perforation, abscess) 3

Diagnostic Criteria on CT

• Appendiceal diameter >6mm (sensitivity 97.5%, specificity 59.6%)
• Optimal cutoff of 8.2mm (sensitivity 88.8%, specificity 93.4%) 1
• Periappendiceal inflammatory changes
• Wall enhancement with IV contrast
• Appendicolith (if present)

Common Pitfalls and Caveats

• Radiation exposure concerns: Low-dose CT protocols maintain similar sensitivity (94%) and specificity (94%) compared to standard-dose protocols 1
• Contrast allergy: In patients with contrast allergy, non-contrast CT can be performed with slightly reduced sensitivity (91%) 2
• Pregnant patients: Ultrasound should be first-line, followed by MRI if equivocal
• Retrocecal appendix: May be difficult to visualize on ultrasound but well-visualized on CT
• Perforated appendicitis: CT has higher sensitivity than ultrasound for detecting perforation 3

Evidence Quality Assessment

The recommendations are based on high-quality evidence from the American College of Radiology guidelines 3 and a comprehensive Cochrane systematic review 2. The Cochrane review included 64 studies with 10,280 participants, providing robust evidence for CT's diagnostic accuracy. The evidence consistently demonstrates the superior performance of CT with IV contrast for diagnosing acute appendicitis in adults.