From the Guidelines
For a patient reporting dark blood in stool, I would recommend a CTA Abdomen and Pelvis Without and With IV Contrast. The presence of dark blood in stool (melena) typically indicates upper gastrointestinal bleeding, and the use of both noncontrast and contrast-enhanced images can help with the detection of a source for gastrointestinal bleeding (GIB) 1. Noncontrast images are useful for the detection of intraluminal high-attenuation material that may mimic intraluminal blood on contrast-enhanced images and may be necessary for the identification of sentinel clot 1.
The addition of IV contrast can help identify active bleeding sites, vascular malformations, tumors, or inflammatory conditions that may be causing the bleeding. A meta-analysis of 22 studies evaluating the accuracy of CTA for the diagnosis of active GIB found that CTA had a sensitivity and specificity of 85% and 92%, respectively 1. Furthermore, multiphasic CT protocols (unenhanced + arterial + portal venous phase) have been shown to have the highest sensitivity of 92% for the detection of GIB compared with unenhanced phase with arterial or portal venous phase alone 1.
Some key considerations when ordering this CT scan include:
- The use of oral contrast is usually not recommended for GIB studies because a positive oral contrast will render the examination nondiagnostic, and oral water can dilute intraluminal hemorrhage 1.
- Patients with significant renal impairment, severe contrast allergy, or those who are pregnant may require alternative imaging approaches, such as non-contrast CT, although this provides less diagnostic information for bleeding sources.
- The results of the CT scan should be interpreted in the context of the patient's overall clinical presentation and may need to be followed up with endoscopy (either upper endoscopy or colonoscopy depending on clinical suspicion) for direct visualization and potential treatment of GI bleeding sources.
From the Research
Diagnostic Approach for Gastrointestinal Bleeding
The patient's report of dark blood in stool suggests gastrointestinal bleeding, which can be diagnosed using various imaging techniques.
- The choice between CT with contrast and without contrast depends on the patient's renal function and allergy status to iodinated contrast material (ICM) 2.
- For patients with renal disease or contrast allergy, alternative contrast media such as carbon dioxide, gadolinium, or dilute ICM can be used 2.
- In cases of occult gastrointestinal bleeding, the initial work-up typically involves colonoscopy or esophagogastroduodenoscopy, or both 3.
- CT scans can be useful in diagnosing gastrointestinal bleeding, especially in cases of active bleeding, but the use of contrast may be limited in patients with renal disease or contrast allergy 3, 4.
Considerations for CT Scan
- If the patient has a history of renal disease or contrast allergy, a CT scan without contrast may be preferred to avoid potential complications 2.
- However, if the patient has no known renal disease or contrast allergy, a CT scan with contrast may be more sensitive in detecting the source of bleeding 3, 4.
- The decision to use contrast or not should be made on a case-by-case basis, taking into account the patient's individual risk factors and medical history 5, 6.