Diagnostic Imaging for Acute Appendicitis

First-Line Imaging Modalities

The American College of Radiology and the Infectious Diseases Society of America recommend ultrasound as the initial imaging modality for children and pregnant patients, while contrast-enhanced CT scan is preferred for adults, with a sensitivity of 95-98% and specificity of 94-95% 1, 2
For non-pregnant adults, abdominal CT scan is the preferred initial imaging modality, with a sensitivity of 95% and specificity of 94%, as recommended by the American College of Radiology and supported by the IDSA 1, 2
For pregnant patients and children, abdominal ultrasound is the preferred initial imaging modality, with MRI as the second-line imaging modality if ultrasound is equivocal, to avoid radiation exposure 2, 3
Ultrasound may be preferred for women of childbearing age to minimize radiation risk, with a conditional recommendation and very low certainty of evidence from the IDSA 2

The World Society of Emergency Surgery strongly recommends using contrast-enhanced low-dose CT scan over contrast-enhanced standard-dose CT scan for adolescents and young adults with suspected acute appendicitis and negative US findings 4
Low-dose CT protocols maintain diagnostic accuracy while reducing radiation exposure, and should be used when possible, especially for younger patients 1
CT without IV contrast and reduced-dose CT protocols can minimize radiation exposure while maintaining diagnostic accuracy, as suggested by the American College of Radiology 1, 2
The following table summarizes the characteristics of imaging modalities:
| Modality | Sensitivity | Specificity | Radiation Exposure | | --- | --- | --- | --- | | CT | 95% | 94% | Yes | | Ultrasound | variable | variable | No | | MRI | high | high | No |

Ultrasound has limitations, including being operator-dependent, limited by patient factors such as obesity and bowel gas, and often yielding equivocal results 3
Contrast-enhanced CT is superior for detecting complications, such as perforated appendicitis, with improved visualization of appendiceal wall inflammation and periappendiceal tissue 5, 3
Combining USS with CT for select cases, including retrocaecal position, can improve diagnostic accuracy from 83% to 93% (p=0.0484) and sensitivity from 81% to 96% (p=0.0014) when CT is added for difficult cases like retrocaecal appendicitis 4
CT remains the gold standard for adults due to its superior diagnostic performance and ability to identify alternative diagnoses, considering radiation exposure, diagnostic accuracy, availability, and patient characteristics 2, 1

Elevated WBC (>10,000/mm³) has a positive likelihood ratio of 2.47, and WBC >15,000/mm³ has a higher positive likelihood ratio of 3.47, while CRP >10 mg/L has a positive likelihood ratio of 4.24, and combined elevated WBC and CRP provides the strongest evidence (positive likelihood ratio of 23.32) 5
Clinical suspicion can guide management, with very low clinical suspicion allowing for discharge with minimal or no testing, and very high clinical suspicion prompting direct surgical consultation with minimal or no testing 5
Obtain surgical consultation promptly if high clinical suspicion or positive imaging, and consider observation if low clinical suspicion and negative imaging 5
The IDSA advises against skipping imaging, as clinical diagnosis alone has high rates of false positives and negatives, and recommends against proceeding directly to CT, especially in women of childbearing age, to avoid unnecessary radiation 2, 1