What is the recommended initial imaging study for patients with suspected Clostridioides difficile (C. diff) infection?

Recommended Imaging for Suspected Clostridioides difficile Infection

Contrast-enhanced cross-sectional imaging, specifically CT abdomen, is recommended as the initial imaging study for patients with suspected Clostridioides difficile infection when imaging is clinically indicated. 1

Clinical Indications for Imaging in C. difficile

Imaging is not routinely needed for diagnosis of C. difficile infection (CDI) but should be considered in the following scenarios:

• Severe or fulminant disease with systemic symptoms
• Suspected complications (toxic megacolon, perforation, ileus)
• Patients who cannot produce stool specimens for laboratory testing
• Unclear diagnosis or poor response to initial therapy

Diagnostic Algorithm for C. difficile Infection

1. First-line diagnostic test: Stool testing for C. difficile toxins using nucleic acid amplification tests (NAAT) or two-step algorithm starting with glutamate dehydrogenase (GDH) screening followed by toxin assay 1

2. When imaging is indicated:

   • CT abdomen with IV contrast is the recommended initial imaging modality 1
   • Ultrasound may be considered in critically ill patients who cannot be transported to CT 1
   • Flexible sigmoidoscopy may be helpful when there is high clinical suspicion but negative stool assays 1

CT Findings in C. difficile Colitis

The most common CT findings in C. difficile colitis include:

• Colonic wall thickening (most common but non-specific finding)
• "Accordion sign" (oral contrast trapped between thickened haustral folds)
• "Double-halo sign" or "target sign" (varying degrees of attenuation caused by submucosal inflammation)
• Pericolonic stranding
• Ascites 1

CT diagnosis of C. difficile colitis has a sensitivity of 52% and specificity of 93%. Sensitivity increases to 70% when using colonic wall thickness >4mm in conjunction with other findings like wall nodularity, accordion sign, pericolonic stranding, or unexplained ascites 1.

Alternative Imaging Modalities

• Ultrasound: May be useful in critically ill patients who cannot be transported to CT. Findings include thickened colonic wall with heterogeneous echogenicity, narrowing of colonic lumen, and visualization of pseudomembranes as hyperechoic lines covering the mucosa. Intraperitoneal free fluid is seen in >70% of cases 1

• Endoscopy: Flexible sigmoidoscopy may be helpful when stool assays are negative but clinical suspicion remains high. However, colonoscopy should be used sparingly due to increased risk of perforation in fulminant colitis 1

Important Considerations

• Imaging findings alone are not specific for C. difficile and must be correlated with clinical presentation and laboratory testing 2
• For patients with suspected severe CDI who cannot produce stool specimens, perirectal swabs may be an acceptable alternative with high sensitivity (95.7%) and specificity (100%) 1
• Over-reliance on imaging alone can lead to misdiagnosis, as wall thickening is non-specific 2
• Empirical therapy for CDI should be considered while awaiting test results in patients with strong suspicion for severe disease 1

Pitfalls to Avoid

• Relying solely on imaging for diagnosis without confirmatory stool testing
• Failure to consider other causes of colonic wall thickening (inflammatory bowel disease, other infectious colitis, ischemic colitis, malignancy)
• Delaying appropriate treatment while waiting for imaging in severely ill patients
• Using colonoscopy in patients with fulminant colitis due to increased perforation risk

By following this approach, clinicians can appropriately utilize imaging in the diagnosis and management of C. difficile infection while avoiding unnecessary tests and potential complications.