What are the indications for a computed tomography (CT) scan in patients presenting with rectal bleeding?

Indications for CT in Rectal Bleeding

In patients with rectal bleeding, CT angiography (CTA) should be the first-line investigation for hemodynamically unstable patients (shock index >1) or those with suspected active bleeding, as it provides the fastest and least invasive means to localize the bleeding site before planning endoscopic or radiological therapy. 1

Patient Assessment and Risk Stratification

When evaluating a patient with rectal bleeding, consider:

• Hemodynamic stability (vital signs, shock index)
• Severity of bleeding (amount, frequency)
• Suspected source (upper vs. lower GI tract)

Specific Indications for CT:

1. Hemodynamically unstable patients:

   • Shock index (heart rate/systolic BP) >1 after initial resuscitation 1
   • Active bleeding with hemodynamic compromise 1
   • Inability to tolerate or undergo colonoscopy due to instability 2

2. Failed endoscopic diagnosis:

   • When endoscopy fails to identify a bleeding source 2
   • After negative upper and lower endoscopy with continued bleeding 1

3. Suspected specific conditions:

   • Anorectal varices with failed detection at endoscopy 1
   • Suspected vascular malformations 2
   • Suspected complicated rectal prolapse 1

CT Protocol Considerations

For optimal detection of rectal bleeding sources, CTA should include:

• Multiphase acquisition 1:

  • Non-contrast phase (or virtual non-contrast if using dual-energy CT)
  • Late arterial phase (35 seconds after contrast injection)
  • Portal venous phase (60-70 seconds after injection)

• No oral contrast (may obscure bleeding) 1

Modern multidetector CT scanners can detect bleeding rates as low as 0.1 mL/min, significantly improved from historical sensitivity of 0.5 mL/min 1.

Diagnostic Performance

CTA demonstrates:

• Sensitivity: 85-90%
• Specificity: 92%
• Accuracy: 94-95% 1

A negative CTA is associated with decreased rates of rebleeding and need for intervention 1.

Algorithm for Rectal Bleeding Evaluation

1. Initial assessment: Evaluate hemodynamic status and shock index

2. Hemodynamically unstable OR shock index >1:

   • Perform immediate CTA without delay 1
   • If CTA negative but high suspicion of upper GI source, proceed to urgent upper endoscopy 1

3. Hemodynamically stable:

   • Proceed with endoscopy (ano-proctoscopy, flexible sigmoidoscopy, or colonoscopy) as first-line diagnostic tool 1
   • Consider CTA if endoscopic evaluation is negative but bleeding continues 2

4. Special populations:

   • In pregnant patients with failed ultrasound detection, consider MRI angiography if available 1

Clinical Pearls and Pitfalls

• Pearl: CTA provides valuable information about vascular anatomy that can guide subsequent interventional procedures 1

• Pitfall: Bright red rectal bleeding with hemodynamic instability may indicate an upper GI source. Consider upper endoscopy before or after CTA in these cases 1

• Pitfall: Avoid delaying appropriate treatment to perform imaging in severely unstable patients 1

• Pearl: CT can detect both the source and cause of active bleeding, expediting treatment initiation 3

• Caution: Consider contrast allergy and renal function before performing CTA 1