What are the implications of ordering a CT scan of the abdomen and pelvis (CTAP) with contrast instead of a CT angiography of the abdomen and pelvis (CTA AP) for evaluation of a lower gastrointestinal bleed (LGIB)?

From the Guidelines

For evaluation of lower gastrointestinal bleeding (LGIB), a CT angiography of the abdomen and pelvis (CTA AP) is the preferred imaging modality, and the ordered CTAP with contrast should be changed to CTA AP to maximize the chance of identifying the bleeding source. The key difference between CTA AP and CTAP with contrast lies in the timing and technique of contrast administration, with CTA AP involving rapid injection of contrast to capture the arterial phase, which is essential for identifying active bleeding sites 1. This technique optimizes visualization of bleeding vessels with acquisition timed to the arterial phase, typically 20-30 seconds after contrast injection. In contrast, CTAP with contrast typically captures the portal venous phase, which may miss active bleeding.

Some key points to consider when evaluating LGIB with CTA AP include:

• The arterial phase imaging is crucial because active extravasation of contrast into the bowel lumen is the key finding that helps localize LGIB, potentially avoiding more invasive diagnostic procedures if the bleeding source is identified 1.
• CTA AP has a high sensitivity and specificity for detecting active bleeding, with values ranging from 79% to 100% and 50% to 100%, respectively 1.
• A negative CTA AP is a good indicator that conservative management can be initiated, with approximately three-fourths of patients settling spontaneously with conservative management when CTA AP is negative 1.
• Transcatheter arteriography immediately after negative CTA AP should be avoided in hemodynamically stable patients, but management of unstable patients with negative CTA AP results remains challenging and should be approached on a case-by-case basis 1.

Overall, CTA AP is the preferred imaging modality for evaluating LGIB, and the ordered CTAP with contrast should be changed to CTA AP to maximize the chance of identifying the bleeding source and guiding appropriate management 1.

From the Research

Evaluation of Lower Gastrointestinal Bleeding (LGIB)

• LGIB is a common cause of hospitalization and has substantial associated morbidity and financial cost 2.
• The incidence of LGIB increases with age, and the most common etiologies are diverticulosis, angiodysplasia, malignancy, and anorectal diseases 3.

Diagnostic Modalities for LGIB

• Computed tomography angiography (CTA) is the imaging modality of choice in those with severe bleeding to diagnose the source of bleeding and guide management when embolization is indicated 4.
• CTA can be used to identify active bleeding in 38% of cases, and the site of bleeding is localized with CTA in a significantly higher proportion of studies compared to tagged RBC scintigraphy 2.
• Colonoscopy is the diagnostic and therapeutic procedure of choice for acute and chronic bleeding 5.
• Angiography is used if colonoscopy fails or cannot be performed, and the use of radioisotope scans is reserved for cases of unexplained intermittent bleeding 5.

CT-Angiography for LGIB

• Multidetector computed tomography (MDCT), or CT-angiography (CT-A), may offer a very sensitive means to evaluate the source of acute LGIB, while avoiding some of the morbidity and intense resource use of contrast angiography 6.
• CT-A may provide unique morphologic information regarding the type of pathology and can be used as a screening tool, followed by either directed therapeutic angiography or surgical management 6.

Ordered CTAP with Contrast Instead of CTA AP

• There is no direct evidence to suggest that CTAP with contrast is the preferred modality for evaluating LGIB, and CTA AP may be more appropriate for diagnosing the source of bleeding and guiding management 4, 2.