In an 11‑month‑old infant with acute painless bright‑red gastrointestinal bleeding mixed with foul‑smelling melena, severe anemia and tachycardia but normal coagulation studies, which investigation is most likely to confirm the bleeding source?

Radioisotope Imaging (Meckel Scan) is Most Likely to Confirm the Bleeding Source

In this 11-month-old infant with painless bright red rectal bleeding, severe anemia, and hemodynamic compromise, radioisotope imaging with Tc-99m pertechnetate (Meckel scan) is the diagnostic test of choice to identify ectopic gastric mucosa in a Meckel diverticulum, the most common cause of significant lower GI bleeding in this age group.

Clinical Context and Differential Diagnosis

This presentation is classic for Meckel diverticulum with ectopic gastric mucosa:

• Painless rectal bleeding is the hallmark presentation in young children, particularly those under 2 years of age 1, 2
• The combination of bright red blood mixed with dark melena suggests a small bowel source with brisk bleeding 1
• Severe anemia (Hb 70 g/L) with tachycardia (HR 160) indicates significant blood loss requiring urgent diagnosis 1
• Meckel diverticulum accounts for >50% of unexplained lower GI bleeding in the pediatric population 3
• Maximum clustering of positive cases occurs at age 1-2 years (52.38% of cases) 2

Why Radioisotope Imaging (Meckel Scan) is the Answer

Diagnostic Performance in Pediatric Patients

• Tc-99m pertechnetate scintigraphy detects ectopic gastric mucosa with a sensitivity of 89% and specificity of 98% in symptomatic patients 1
• The test is 84.2% true positive for Meckel diverticulum in pediatric patients with GI bleeding 2
• Positive and negative predictive values approach 100% in anemic patients with GI bleeding 1

Mechanism and Technical Advantages

• Tc-99m pertechnetate is actively taken up by gastric mucosa (both normal and ectopic), creating a fixed focal area of uptake that appears simultaneously with or before the stomach 1, 4
• The scan can detect ectopic gastric mucosa even when bleeding is not actively occurring at the time of imaging 1
• This is critical because bleeding from Meckel diverticulum is often intermittent, making it difficult to capture with other modalities that require active hemorrhage 5

Clinical Algorithm Support

• Meckel scan is the diagnostic test of choice for young patients with painless rectal bleeding after negative upper endoscopy 1
• The American College of Radiology recommends Meckel scan as appropriate in young patients with unexplained lower GI bleeding 1
• The test should be performed with H2-receptor blocker premedication (cimetidine or ranitidine) to enhance sensitivity by reducing secretion and washout of the radiotracer 6

Why Other Options Are Less Appropriate

Abdominal Ultrasonography (Option A)

• Ultrasound has limited sensitivity for detecting Meckel diverticulum unless complicated by intussusception or perforation 1
• Cannot reliably identify ectopic gastric mucosa, which is the key pathophysiologic feature causing bleeding 1

Barium Enema (Option C)

• Barium studies have no role in acute GI bleeding as they obscure active hemorrhage and interfere with subsequent diagnostic procedures 5, 7
• Cannot detect ectopic gastric mucosa 1
• May delay definitive diagnosis and treatment 5

Barium Meal (Option D)

• Similarly contraindicated in acute bleeding scenarios 5, 7
• Does not evaluate the distal ileum where Meckel diverticulum is located (typically within 100 cm of the ileocecal valve) 1

Critical Pitfalls to Avoid

False Positives and Atypical Findings

• Physiologic activity in the ureters, bladder, or other structures can mimic ectopic gastric mucosa 8
• Atypical findings may include perforated appendix, calyceal stasis, or regional enteritis 4
• SPECT or SPECT/CT imaging should be used to improve localization accuracy 8, 9

Timing and Technique

• Acquire delayed images if initial images are equivocal, as ectopic tissue may show delayed uptake 4
• Administer furosemide to clear bladder activity that may obscure pelvic lesions 6
• Ensure 48-hour premedication with H2-blockers for optimal sensitivity 6

Hemodynamic Considerations

• While this patient is tachycardic, the blood pressure (118/56 mmHg) suggests relative hemodynamic stability appropriate for nuclear medicine imaging 5, 8
• If the patient becomes hemodynamically unstable with ongoing massive bleeding, proceed directly to angiography or surgery rather than scintigraphy 5

Confirmatory Testing

• Even with a positive Meckel scan, surgical exploration is required for definitive diagnosis and treatment 5, 2
• Histopathologic confirmation of ectopic gastric mucosa should be obtained 2, 6
• In the 19 surgically confirmed cases from one series, 16 were true positives for Meckel diverticulum 2