What is the radiological investigation of choice for diagnosing appendicitis?

Radiological Investigation of Choice in Appendicitis

CT of the abdomen and pelvis with intravenous contrast is the radiological investigation of choice for diagnosing appendicitis in adults, achieving sensitivities of 85.7-100% and specificities of 94.8-100%. 1

Primary Imaging Recommendation

For non-pregnant adults with suspected appendicitis, proceed directly to CT abdomen and pelvis with IV contrast as the first-line imaging modality. 2 This approach has:

• Sensitivity: 95% (95% CI: 0.93-0.96) 3
• Specificity: 94% (95% CI: 0.92-0.95) 3
• Negative appendectomy rate: 1.7-7.7% (compared to 14.7% historically without imaging) 1
• Rated 8/9 on ACR Appropriateness Criteria 2

Contrast Protocol Selection

Use IV contrast alone without oral or rectal contrast to optimize both diagnostic accuracy and time efficiency. 4

• IV contrast-enhanced CT achieves 100% sensitivity and 98.6% specificity 4
• Oral contrast adds no significant diagnostic benefit but delays ED disposition by 1.5 hours and time to OR by 1 hour 4
• Rectal contrast does not increase accuracy compared to IV contrast alone 1
• IV contrast enhancement provides superior sensitivity (0.96) compared to unenhanced CT (0.91) 3

Radiation Dose Considerations

Low-dose CT protocols are equivalent to standard-dose CT for appendicitis diagnosis and should be utilized when available. 1

• Low-dose CT achieves sensitivity of 0.94 (95% CI: 0.90-0.97) with no difference in specificity compared to standard-dose 1
• Radiation exposure reduced to approximately 2 mSv (compared to 10 mSv for standard-dose) 2
• Non-inferiority demonstrated in randomized trials of 891 patients 1

Alternative Imaging Strategies

Ultrasound as Initial Screening

Ultrasound may be used as initial screening (rated 6/9 on ACR criteria) followed by CT if inconclusive, particularly in resource-limited settings or to reduce radiation exposure. 2

• Ultrasound has high specificity (85-89%) but low sensitivity (33-35%) for appendicitis 5
• Major limitation: poor inter-observer reliability (kappa = 0.15-0.20) 5
• Cannot reliably exclude appendicitis when negative, necessitating CT follow-up 5
• Low sensitivity for perforated appendicitis should be considered if non-operative management is contemplated 1

MRI for Special Populations

For pregnant women, use MRI abdomen and pelvis without IV contrast as the primary modality to avoid ionizing radiation. 2

• MRI achieves 100% sensitivity and 98% specificity in pregnant patients 2
• Appendix visualization rate: 80% for MRI versus 7% for ultrasound in pregnancy 2
• Unenhanced MRI: sensitivity 86%, specificity 94%; with IV contrast: sensitivity 94%, specificity 94% 1
• Diffusion-weighted imaging sequences increase conspicuity and reader confidence 1

Common Pitfalls to Avoid

Do not delay CT imaging waiting for oral contrast administration in patients with high clinical suspicion, as this increases time to diagnosis without improving accuracy. 4

Do not rely on ultrasound alone to exclude appendicitis due to its low sensitivity (33-35%) and poor inter-observer agreement. 5

Do not order unenhanced CT as first-line imaging when IV contrast is available, as sensitivity drops from 96% to 91%. 3

Do not use plain radiography as it has minimal diagnostic value for appendicitis evaluation. 2

Ensure 24-hour follow-up for patients with negative imaging but persistent symptoms due to risk of false-negative results. 6