What is the most appropriate investigation to confirm the source of gastrointestinal bleeding in an infant with hemoglobin level of 70 g/L, presenting with bright red blood and dark brownish-black stool?

Radioisotope Imaging is the Most Appropriate Investigation

In an 11-month-old infant presenting with bright red blood mixed with dark stool, severe anemia (Hb 70 g/L), and hemodynamic instability (tachycardia), radioisotope imaging with Tc-99m-labeled RBC scan is the most appropriate investigation to confirm the source of bleeding, as it can detect bleeding rates as low as 0.05-0.1 mL/min and is particularly useful for identifying Meckel diverticulum, the most common cause of significant lower GI bleeding in this age group. 1

Clinical Presentation Analysis

This infant presents with:

• Mixed bleeding pattern: Bright red blood (hematochezia) combined with dark brownish-black stool (melena) suggests a lower GI source with brisk bleeding 1
• Severe anemia: Hemoglobin of 70 g/L (normal 112-165 g/L) indicates significant blood loss 1
• Hemodynamic compromise: Heart rate of 160/min (tachycardia) with pallor suggests ongoing or recent substantial hemorrhage 1
• Age-specific consideration: At 11 months, Meckel diverticulum is the leading cause of painless lower GI bleeding requiring investigation 1

Why Radioisotope Imaging is Superior

Sensitivity Advantages

• Tc-99m-labeled RBC scanning detects bleeding rates as low as 0.05-0.1 mL/min, making it more sensitive than CT angiography (0.3 mL/min) and conventional angiography (0.5 mL/min) 1, 2
• This superior sensitivity is critical in pediatric patients where bleeding may be intermittent or at lower rates 1
• SPECT/CT imaging improves anatomical localization and reduces historical localization errors associated with planar imaging 1

Age-Appropriate Diagnosis

• In infants with severe anemia and hemodynamic instability, radioisotope imaging is particularly useful for identifying Meckel diverticulum, which requires urgent surgical intervention 1
• The diagnostic algorithm should start with radioisotope imaging to localize the bleeding source, followed by surgical consultation for resection if positive 1

Why Other Options Are Inappropriate

Barium Studies (Options C & D) - Contraindicated

• Barium studies are contraindicated in acute GI bleeding evaluation as they obscure active hemorrhage and interfere with subsequent endoscopy or angiography 1, 2
• The American College of Radiology explicitly states that fluoroscopy with barium has no role in acute GI bleeding evaluation 2
• Barium masks contrast extravasation and compromises subsequent diagnostic and therapeutic procedures 2

Abdominal Ultrasonography (Option A) - Limited Utility

• While ultrasound may identify intussusception (another cause of bloody stool in infants), the clinical presentation here—with severe anemia, hemodynamic instability, and mixed bleeding pattern—is more consistent with Meckel diverticulum or other bleeding sources that ultrasound cannot adequately evaluate 1
• Ultrasound does not detect active bleeding or localize bleeding sources effectively 1

Diagnostic Algorithm for This Patient

1. Immediate stabilization: Address hemodynamic instability with fluid resuscitation and blood transfusion as needed 1
2. Radioisotope imaging (Tc-99m-labeled RBC scan): First-line investigation to localize bleeding source 1
3. If positive for Meckel diverticulum: Urgent surgical consultation for resection 1
4. If initial study negative but bleeding continues: Consider repeat imaging, as intermittent bleeding may require multiple studies 1
5. Second-line options: CTA in hemodynamically stable patients if nuclear medicine is negative or unavailable 1
6. Colonoscopy: May be considered if other studies are non-diagnostic, though challenging in this age group 1

Critical Pitfalls to Avoid

• Do not order barium studies in acute GI bleeding, as they will prevent all subsequent diagnostic and therapeutic interventions 1, 2
• Recognize that intermittent bleeding may require repeat imaging if the initial study is negative, and this possibility is crucial for timely diagnosis and treatment 1
• Do not delay imaging for endoscopy in this age group, as the bleeding source is likely in the small bowel (Meckel diverticulum), which is not accessible by standard endoscopy 1

The answer is B. Radioisotope imaging.