What is the normal diameter of the appendix?

Normal Appendix Diameter

The normal appendix diameter is typically less than 6 mm, with measurements between 6-8 mm considered borderline, and greater than 8 mm highly suggestive of appendicitis. 1

Diagnostic Criteria for Appendix Diameter

Normal Range

• The normal appendix typically measures less than 6 mm in maximum outer diameter (MOD)
• Mean diameter of normal appendix: 6.3 mm (range: 2.8-11.3 mm) 2
• In a large retrospective study of 1,575 patients with non-appendicular symptoms, 48.12% had an appendix diameter >6 mm 2

Borderline Measurements

• Appendices measuring 6-8 mm fall into a "borderline" category that requires additional evaluation
• These measurements are neither clearly normal nor abnormal when diagnosis is based solely on diameter 3
• Additional sonographic findings should be considered for appendices in this size range

Diagnostic Thresholds

• Different cutoff values provide varying sensitivity and specificity:
  • 6 mm cutoff: 97.5% sensitivity, 59.6% specificity, 73.6% accuracy 1
  • 6.6 mm cutoff: 93.8% sensitivity, 94.9% specificity, 94.4% accuracy 1
  • 8.2 mm cutoff: 88.8% sensitivity, 93.4% specificity, 91.7% accuracy 1
  • 8.5 mm cutoff: 90.2% sensitivity, 91.5% specificity 1

Imaging Modality Considerations

CT vs. Ultrasound Measurements

• CT measurements are consistently larger than ultrasound measurements by 1-2 mm 4
• For normal appendices: Mean difference of 1.3-1.4 mm (CT: 5.3 ± 1.0 mm vs. US: 4.7 ± 1.1 mm) 4
• For inflamed appendices: Mean difference of 2 mm (CT: 8.3 ± 2.2 mm vs. US: 7.0 ± 2.0 mm) 4
• This difference calls into question using the same diameter cutoff (6 mm) for both modalities 4

Optimal Diagnostic Approach

• For CT imaging: A cutoff between 8-9 mm provides the best balance of sensitivity (84%) and specificity (87%) 5
• For ultrasound: A cutoff of 6 mm is traditionally used, but should be interpreted with caution 6
• In adults with persistent right lower quadrant pain, an appendix >6 mm on ultrasound suggests appendicitis 6

Additional Diagnostic Considerations

Beyond Diameter Alone

• Diameter with compression (MOD minus compressible contents) may be more accurate than diameter alone 1
• Additional findings that improve diagnostic accuracy:
  • Appendiceal wall enhancement on CT 5
  • Wall thickness >3 mm 1
  • Periappendiceal fat stranding 1
  • Absence of intraluminal gas 1

Clinical Pitfalls

• Relying solely on the 6 mm cutoff can lead to overdiagnosis and unnecessary appendectomies 2
• In adults with appendices ≤6 mm and no appendicolith, observation rather than immediate surgery is recommended 6
• The presence of appendicoliths increases risk of appendicitis even with borderline measurements 6

Conclusion

When evaluating appendiceal diameter, consider the imaging modality used, as CT measurements are typically 1-2 mm larger than ultrasound measurements. While 6 mm has traditionally been used as the cutoff, higher thresholds (particularly for CT) provide better specificity without significantly compromising sensitivity. For optimal diagnostic accuracy, appendiceal diameter should be considered alongside other imaging findings and clinical presentation.