CT Imaging for Extremity Cellulitis
For uncomplicated cellulitis of an extremity, CT imaging is generally not indicated and represents potential overuse, as cellulitis is primarily a clinical diagnosis. 1, 2
When CT May Be Appropriate
If imaging is deemed necessary to evaluate for complications or when diagnosis is uncertain, CT of the area of interest WITH intravenous contrast is the preferred CT protocol (rated 6/9 - "may be appropriate" by ACR guidelines). 1
Key Indications for CT in Extremity Cellulitis:
- Suspected deep infection or abscess formation - Contrast helps delineate fluid collections and areas of necrosis 1
- Concern for necrotizing fasciitis - CT with IV contrast shows fascial thickening and lack of fascial enhancement, which are highly specific findings 1, 2
- Suspected underlying osteomyelitis - When there is a skin lesion, wound, ulcer, or blister with cellulitis (rated 7/9 - "usually appropriate") 1
- History of recent trauma - Independent predictor of positive findings (relative risk 6.24) 3
- Presence of chronic wound - Independent predictor of positive findings (relative risk 2.98) 3
Important Clinical Caveats
Low Yield in Uncomplicated Cases:
- A 2025 single-center study found CT yield for deep infection in lower extremity cellulitis was only 5.5%, significantly lower than the 10% threshold for clinical utility 4
- Among 496 encounters, only 12.1% proceeded to surgery, with just 21 cases preceded by positive CT findings 4
- Patients without trauma history or chronic wounds do not warrant routine imaging 3
Contrast vs. Non-Contrast:
- Contrast is strongly preferred for soft-tissue evaluation as it improves detection of abscesses, tissue necrosis, and vascular complications 1
- Non-contrast CT (rated 4/9 - "usually not appropriate") has limited utility except for detecting soft tissue gas (89% sensitivity, 93% specificity) 1
- Contrast may obscure foreign body identification if that is the primary concern 1
Alternative Imaging Approaches
Before ordering CT, consider these more appropriate first-line modalities:
- Ultrasound - Rated 5/9 for juxta-articular regions, useful for detecting fluid collections and abscesses, can be performed at bedside 1
- Plain radiographs - Should be obtained first to evaluate for gas, foreign bodies, or underlying bone abnormalities, though they should not be used to rule out necrotizing infection 1, 2
- MRI with and without IV contrast - Rated 9/9 ("usually appropriate") when osteomyelitis is suspected, superior for soft tissue detail but may not be practical in emergency settings 1
Critical Warning
Imaging studies should NEVER delay surgical consultation and intervention when necrotizing fasciitis is clinically suspected - this is a life-threatening emergency with mortality rates of 29-80%. 1, 2