Do I order a nuclear tagged red blood cell (RBC) scan for a patient with a gastrointestinal (GI) bleed?

Nuclear Medicine Tagged RBC Scan for GI Bleeding: When to Order

For patients with GI bleeding, CT angiography (CTA) should be the first-line imaging study for hemodynamically unstable patients or those with suspected active bleeding, while nuclear medicine tagged RBC scans should be reserved for hemodynamically stable patients with intermittent or slow bleeding when other modalities have failed to identify the source. 1

Initial Approach to GI Bleeding

Hemodynamically Unstable Patients

• First-line imaging: CT angiography (CTA)
  • Faster acquisition time
  • Can detect bleeding rates as low as 0.3 mL/min
  • Provides anatomical information to guide intervention
  • Should include unenhanced and contrast-enhanced phases 1

Hemodynamically Stable Patients

1. First perform endoscopy (upper or lower based on suspected source)
2. If endoscopy is negative or inconclusive:
   • For suspected active bleeding: CTA
   • For suspected intermittent/slow bleeding: Tagged RBC scan

Role of Nuclear Medicine Tagged RBC Scan

Appropriate Indications

• Hemodynamically stable patients
• Intermittent or slow bleeding (0.04-0.1 mL/min)
• Negative endoscopy and CTA
• When the bleeding source remains unidentified after other modalities 1

Advantages of Tagged RBC Scan

• High sensitivity for slow or intermittent bleeding (93%)
• Can detect bleeding rates as low as 0.04 mL/min
• Allows prolonged imaging (up to several hours)
• Useful when bleeding is intermittent 1

Limitations of Tagged RBC Scan

• Lower accuracy (55.4%) compared to CTA (96%) 1
• Less precise anatomical localization
• Time-consuming (requires at least 1 hour of imaging)
• Delayed diagnosis compared to CTA
• Not suitable for hemodynamically unstable patients 1

Optimal Protocol for Tagged RBC Scan

1. In vitro RBC labeling method (highest labeling efficiency)
2. Flow images (angiographic phase): 1-2 seconds per frame for 1 minute
3. Dynamic imaging: 1 minute per frame for minimum 1 hour
4. Review images in 10-15 minute sequences during acquisition
5. Consider SPECT/CT to clarify indeterminate findings 1

Important Considerations

Predictive Value

• Early positive scans (≤9 minutes) have higher likelihood of positive findings on subsequent catheter angiography
• Negative scans are associated with decreased rates of rebleeding and need for intervention 1

Common Pitfalls

• Waiting too long between a positive RBC scan and catheter angiography reduces diagnostic yield
• Relying solely on planar imaging without SPECT/CT may reduce accuracy
• Using RBC scan as first-line imaging in unstable patients delays definitive treatment 1

Algorithm for GI Bleeding Evaluation

1. Assess hemodynamic stability
2. If unstable: Resuscitate → CTA → Catheter angiography if positive
3. If stable:
   • Perform endoscopy (upper or lower based on clinical presentation)
   • If negative and active bleeding suspected: CTA
   • If negative and intermittent/slow bleeding suspected: Tagged RBC scan
4. If RBC scan positive: Proceed to catheter angiography or targeted endoscopy based on localization

Remember that the British Society of Gastroenterology guidelines specifically state that nuclear medicine may offer diagnosis when other investigations like CTA, angiography, or colonoscopy are negative, particularly when bleed rates are intermittent or slow 1.