What is the most appropriate diagnostic test for a 1-month-old baby with constipation, abdominal distension, and bilious vomiting since birth?

Diagnostic Test for Neonatal Distal Bowel Obstruction

The highest diagnostic value test is a plain abdominal X-ray (Option A), which should be performed first as the initial imaging study, followed by contrast enema if distal obstruction is confirmed—not rectal biopsy as the initial diagnostic test. 1, 2

Initial Diagnostic Approach

Plain abdominal radiographs are the first-line imaging study for all suspected bowel obstruction in infants, as they demonstrate critical findings including dilated bowel loops, air-fluid levels, and the presence or absence of distal gas. 1, 2 This clinical presentation—constipation since birth, abdominal distension, and bilious vomiting—is classic for distal bowel obstruction and requires urgent evaluation. 3, 4

Why Plain X-ray First

• Plain films provide immediate diagnostic information that guides the next step in management, showing whether obstruction is proximal versus distal, and the pattern of gas distribution. 1
• The American College of Radiology explicitly recommends plain abdominal radiographs as the first imaging study for suspected bowel obstruction. 1, 2
• Multiple dilated loops with absent or decreased distal gas indicate distal obstructions such as ileal atresia or Hirschsprung's disease. 1

The Critical Pitfall to Avoid

Do not proceed directly to rectal biopsy (Option B) without imaging first. 5 The American College of Radiology specifically advises against assuming Hirschsprung disease and proceeding directly to rectal biopsy, as the clinical presentation is identical for multiple causes of distal obstruction including meconium plug, ileal atresia, meconium ileus, and Hirschsprung disease. 5

Why Rectal Biopsy Is Not First-Line

• While rectal biopsy is the definitive test for Hirschsprung disease, it is invasive and should only be performed after imaging confirms the need. 5, 6
• Only 22% of children referred for rectal biopsy for suspected Hirschsprung disease actually have the condition, demonstrating significant overuse when proper imaging algorithms are not followed. 6
• Contrast enema is both diagnostic AND potentially therapeutic, and must be performed before considering biopsy. 5

The Algorithmic Approach

1. Plain abdominal X-ray first to identify obstruction pattern and level 1, 2
2. Contrast enema second if distal obstruction is suspected, as it can differentiate between meconium plug (therapeutic), ileal atresia, meconium ileus, and Hirschsprung disease, and demonstrates the transition zone in Hirschsprung disease 5
3. Rectal biopsy third only if contrast enema findings suggest Hirschsprung disease 5

Why Not the Other Options

• Anal manometry (Option C): Not mentioned in current guidelines as a first-line diagnostic test for neonatal obstruction; it has a role in older children with suspected Hirschsprung disease but requires patient cooperation. 5
• Ultrasound abdomen (Option D): The American College of Radiology explicitly states that ultrasound has no role in evaluating neonates with suspected distal obstruction, as there is no relevant literature supporting its use in this clinical scenario. 5, 1

In this 1-month-old with constipation since birth, abdominal distension, and bilious vomiting, plain X-ray provides the highest initial diagnostic value by establishing the presence and pattern of obstruction, which then directs subsequent definitive testing. 1, 2