What are the typical CT findings of appendicitis?

CT Findings of Appendicitis

The most diagnostically valuable CT findings for appendicitis are an enlarged appendix >8.2 mm in maximum outer diameter (sensitivity 88.8%, specificity 93.4%), appendiceal wall thickening, periappendiceal fat stranding, and appendiceal wall enhancement—with an enlarged appendix plus fat stranding present in 93% of cases. 1

Primary Diagnostic Features

Appendiceal Diameter

• Maximum outer diameter (MOD) >8.2 mm is the optimal cutoff, yielding sensitivity 88.8%, specificity 93.4%, and accuracy 91.7% 1
• The traditional 6 mm cutoff has higher sensitivity (97.5%) but poor specificity (59.6%), resulting in more false positives 1
• Diameter with compression (MOD minus compressible contents) >6.6 mm provides the best overall performance with sensitivity 93.8%, specificity 94.9%, and accuracy 94.4% 1

Wall Abnormalities

• Appendiceal wall thickening demonstrates 66% sensitivity and 96% specificity 1, 2
• Appendiceal wall enhancement shows 75% sensitivity and 85% specificity 2
• Wall thickness >3 mm is significantly associated with perforation (OR 3.2, p=0.02) 1

Periappendiceal Changes

• Periappendiceal fat stranding occurs in 87% of appendicitis cases with 74% specificity 2
• Fat stranding is present in 100% of appendicitis cases in some series, though specificity is only 80% 3
• This finding has high sensitivity but can occur with alternative diagnoses, requiring correlation with other signs 3

Secondary Diagnostic Features

Appendicoliths

• Intraluminal appendicolith has 44-55% sensitivity but 100% specificity 3, 1
• Appendicoliths predict perforation with OR 2.47-2.67 (p=0.015) 1
• Extraluminal appendicolith is highly specific for perforation (OR 28.9, p=0.02) but has low sensitivity 1

Cecal Changes

• Focal cecal apical thickening demonstrates 69% sensitivity and 100% specificity 3
• This finding helps distinguish appendicitis from other right lower quadrant pathology 2

Fluid and Gas

• Periappendiceal fluid with severity grade ≥2.5 has 22% sensitivity but 100% specificity 1
• Absence of intraluminal gas is present in 67.6% of appendicitis versus 48.9% without (p=0.024) 1
• Intraluminal air predicts perforation with OR 2.64 (sensitivity 36.9%, specificity 81.9%) 1

Diagnostic Algorithm for Complicated Appendicitis

Signs of Perforation/Necrosis

• Extraluminal gas is the strongest predictor (OR 28.9, p=0.02) with 96% specificity but only 38% sensitivity for perforation 1
• Focal wall enhancement defect has the highest specificity at 98.8% for perforation 1
• Circumferential periappendiceal inflammatory changes predict perforation with OR 5.63 (p<0.001) 1
• Appendiceal diameter ≥11 mm has the greatest sensitivity (62.7%) for perforation 1
• Abscess formation indicates established perforation with 100% specificity 1

Multi-Feature Assessment

• Presence of ≥2 CT signs increases the odds ratio of appendicitis to 6.8 (95% CI: 3.0-15.5, p<0.001) 1
• For complicated appendicitis, any of 10 features (wall enhancement defect, abscess, extraluminal air, intraluminal air, extraluminal appendicolith, intraluminal appendicolith, moderate-to-severe fat stranding, periappendiceal fluid, ileus, ascites) provides 92% sensitivity but only 43% specificity 4

Contrast Enhancement Protocols

IV Contrast Performance

• Contrast-enhanced CT has superior sensitivity (96%, 95% CI: 0.92-0.98) compared to unenhanced CT (90%, 95% CI: 0.87-0.93) 1
• Specificity is comparable between contrast-enhanced (93%) and unenhanced (94%) CT 1
• CT with IV and oral contrast shows sensitivity 96% (95% CI: 0.93-0.98) 1

Enteral Contrast Considerations

• CT without enteral contrast demonstrates sensitivity 90-100% and specificity 94.8-100%, comparable to CT with enteral contrast 1, 5
• Rectal contrast provides the highest sensitivity (97%, 95% CI: 0.93-0.99) and specificity (95%, 95% CI: 0.90-0.98) 1
• Oral contrast shows no significant difference versus unenhanced CT 1

Common Pitfalls

• Nonvisualization of the appendix on CT does not exclude appendicitis—16.4% of patients with nonvisualized appendix on ultrasound had appendicitis on subsequent CT 1
• Single sign reliance is inadequate—look for multiple concordant findings 1
• Low specificity of 6 mm cutoff leads to overdiagnosis—use 8.2 mm or diameter with compression instead 1
• Perforation underdiagnosis occurs in 63% of cases when relying on gross CT signs alone—actively search for subtle wall defects and extraluminal gas 1
• Multiplanar reformation improves appendiceal visualization in patients with scanty fat, unusual appendiceal location, or adjacent abscess 6