How does Computed Tomography (CT) diagnose appendicitis?

How CT Determines Appendicitis

CT diagnoses appendicitis primarily by identifying an enlarged appendix (>8.2 mm outer diameter) combined with secondary inflammatory signs including periappendiceal fat stranding, appendiceal wall thickening, and appendiceal wall enhancement on contrast-enhanced imaging. 1

Primary Diagnostic Criterion

The single most important CT finding is appendiceal diameter:

• Optimal cutoff: >8.2 mm outer diameter yields sensitivity 88.8%, specificity 93.4%, and accuracy 91.7% 1
• Alternative measurement: diameter with compression (outer diameter minus compressible contents) >6.6 mm provides even better performance with sensitivity 93.8%, specificity 94.9%, and accuracy 94.4% 1
• The traditional 6 mm cutoff has very high sensitivity (97.5%) but poor specificity (59.6%), leading to overdiagnosis 1
• Appendiceal diameter alone is more accurate than combining all secondary signs together 2

Secondary Diagnostic Features

Four key secondary findings significantly improve diagnostic confidence when present alongside appendiceal enlargement:

Most Useful Secondary Signs

• Periappendiceal fat stranding: sensitivity 87%, specificity 74% 3
• Appendiceal wall thickening (>3 mm): sensitivity 66%, specificity 96% 3
• Appendiceal wall enhancement: sensitivity 75%, specificity 85% 3
• Focal cecal apical thickening: statistically significant association with appendicitis 3

Additional Supporting Findings

• Absence of intraluminal gas: present in 67.6% of appendicitis cases versus 48.9% without (p=0.024) 1
• Appendicolith: highly specific but low sensitivity 1
• Periappendiceal fluid: grade ≥2.5 has 100% specificity but only 22% sensitivity 1

Contrast Enhancement Protocols

CT with IV contrast is the preferred imaging technique:

Performance by Contrast Type

• IV contrast alone: sensitivity 96% (95% CI: 0.92-0.98), specificity 94.8-100% 1, 4
• Rectal contrast: highest performance with sensitivity 97%, specificity 97% 1, 4
• Oral + IV contrast: sensitivity 96%, specificity 93% 1, 4
• Noncontrast CT: sensitivity 90-93%, specificity 94-98% 1, 4

Why IV Contrast is Preferred

• Highlights appendiceal wall inflammation and enhancement 1, 5
• Demonstrates periappendiceal inflammatory changes more clearly 1, 5
• Particularly valuable in thin patients with low body mass index who lack sufficient mesenteric fat 1, 5
• Helps identify alternative diagnoses (diverticulitis, inflammatory bowel disease, cancer) 1

Oral Contrast Should Be Avoided

• Does not improve diagnostic accuracy: systematic review of 23 studies showed no benefit 1, 5
• Causes significant delays: requires 40 minutes to 2+ hours for bowel transit 1, 5
• Poorly tolerated by patients with abdominal pain and vomiting 1, 5
• Adds unnecessary cost without diagnostic benefit 1, 5

Identifying Complicated Appendicitis

Five high-specificity CT findings predict perforation and guide surgical urgency:

Highest Specificity Features

• Extraluminal appendicolith: OR 28.9 for perforation, 96% specificity 1, 6, 7
• Focal wall enhancement defect: 98.8% specificity for perforation 1, 7
• Extraluminal air: OR 28.9,96% specificity but only 38% sensitivity 1, 7
• Abscess formation: highly specific for complicated disease 1, 7
• Circumferential periappendiceal inflammatory changes: OR 5.63 for perforation 1, 7

Additional Perforation Predictors

• Appendiceal diameter ≥11 mm: greatest sensitivity (62.7%) for perforation 1, 7
• Wall thickness >3 mm: OR 3.2 for perforation 1, 7

Overall Diagnostic Performance

CT demonstrates excellent accuracy for appendicitis diagnosis:

• Summary sensitivity: 95% (95% CI: 0.93-0.96) 4
• Summary specificity: 94% (95% CI: 0.92-0.95) 4
• Positive predictive value: 92% at median prevalence of 43% 4
• Negative predictive value: 96% (only 4% probability of appendicitis after negative CT) 4
• Negative appendectomy rate with preoperative CT: 1.7-7.7% compared to historical rate of 14.7% without imaging 1

Practical Algorithm for CT Interpretation

Step 1: Measure Appendiceal Diameter

• If >8.2 mm outer diameter → proceed to Step 2 1
• If 6-8.2 mm → evaluate secondary signs carefully 1
• If <6 mm → appendicitis unlikely unless secondary signs strongly present 1

Step 2: Assess Secondary Inflammatory Signs

• Look for ≥2 of the following: fat stranding, wall thickening, wall enhancement, absence of intraluminal gas 1, 3
• When ≥2 secondary signs present: OR 6.8 for appendicitis 1

Step 3: Evaluate for Complications

• Search for extraluminal appendicolith, wall defects, extraluminal air, abscess 1, 6, 7
• If any high-specificity feature present: complicated appendicitis requiring urgent surgical consultation 6, 7

Common Pitfalls and Caveats

Critical Errors to Avoid

• Do not rely on the traditional 6 mm cutoff alone as it has poor specificity (59.6%) and leads to overdiagnosis 1
• Do not dismiss fat stranding as nonspecific in the context of right lower quadrant pain—it strongly supports appendicitis when combined with other findings 6, 3
• Do not assume fat stranding alone indicates perforation—combine with high-specificity features (wall defect, extraluminal air, appendicolith) to accurately stratify risk 6, 7
• Do not delay CT for oral contrast administration—it provides no diagnostic benefit and causes unnecessary delays 1, 5

Special Considerations

• In equivocal cases with negative or borderline CT but high clinical suspicion: consider observation with supportive care or surgical consultation, as CT sensitivity is not 100% 5
• Noncontrast CT remains highly accurate (sensitivity 90-96%, specificity 96-100%) if IV contrast is contraindicated due to severe allergy or renal failure 1, 5, 4
• Low-dose CT protocols show similar diagnostic accuracy to standard-dose CT (sensitivity 94% vs 95%) with significantly reduced radiation exposure 1, 4