What is the recommended initial imaging test to rule out appendicitis?

Recommended Initial Imaging for Suspected Appendicitis

For non-pregnant adults with suspected acute appendicitis, abdominal CT scan is the recommended initial imaging modality to diagnose or rule out appendicitis. 1

Patient-Specific Imaging Recommendations

Non-Pregnant Adults

• First-line imaging: Abdominal CT scan with IV contrast
  • CT without IV contrast also has high diagnostic accuracy and may be appropriate when IV contrast is contraindicated 1
  • High sensitivity (95%) and specificity (94%) for diagnosing appendicitis 2
  • Allows identification of alternative diagnoses that may mimic appendicitis

Children and Adolescents

• First-line imaging: Abdominal ultrasound (US) 1
  • Avoids radiation exposure, which is particularly important in children
  • When US is definitively positive or negative, it has comparable accuracy to CT/MRI
  • If initial US is equivocal/non-diagnostic and clinical suspicion persists:
    • Proceed to MRI or CT as subsequent imaging 1
    • CT with IV contrast is usually appropriate after equivocal US 1

Pregnant Patients

• First-line imaging: Abdominal ultrasound 1
  • Avoids radiation exposure to fetus
  • If initial US is equivocal:
    • Proceed to MRI as subsequent imaging 1
    • MRI may be used as initial imaging if readily available 1

Imaging Performance Characteristics

CT Scan

• Highest overall accuracy in adults
• Sensitivity: 95% (93-96%)
• Specificity: 94% (92-95%) 2
• IV contrast enhancement improves sensitivity (96%) compared to unenhanced CT (91%) 2
• Low-dose CT protocols have similar diagnostic accuracy to standard-dose CT 2

Ultrasound

• When used as second-line imaging after initial US:
  • In children: Sensitivity 91.3%, Specificity 95.2%
  • In adults: Sensitivity 83.1%, Specificity 90.9% 3
• Operator-dependent and can yield equivocal results 1
• Limited in visualizing retrocecal appendix or in obese patients

MRI

• When used as second-line imaging after initial US:
  • In children: Sensitivity 97.4%, Specificity 97.1%
  • In adults: Sensitivity 89.9%, Specificity 93.6% 3
• Excellent option for pregnant patients and children after equivocal US
• IV contrast can be reserved for equivocal non-contrast MRI cases, reducing contrast use by approximately 80% 4

Clinical Pitfalls and Caveats

• If CT is negative but clinical suspicion remains high, consider:

  • Observation and supportive care, with or without antibiotics
  • Surgical intervention if clinical suspicion is very high 1

• For ultrasound:

  • Operator dependency may affect diagnostic accuracy
  • False negatives can occur with retrocecal appendix location or in obese patients
  • When US is equivocal, proceeding directly to a second imaging modality is more effective than repeating US 1

• For MRI:

  • May require sedation in young children
  • Limited availability in some settings, especially for emergency cases
  • Longer acquisition time compared to CT

The choice of imaging modality should consider radiation exposure, diagnostic accuracy, availability, and patient characteristics, but CT remains the gold standard for adults due to its superior diagnostic performance and ability to identify alternative diagnoses.