Characteristic Abdominal Series Findings in Small-Bowel Obstruction
On an abdominal series, you should expect to see dilated small-bowel loops (>2.5-3 cm diameter) with air-fluid levels on upright films, and a relative paucity or complete absence of gas in the colon. 1, 2
Classic Radiographic Findings
The most diagnostically significant findings on abdominal series include:
- Dilated small-bowel loops exceeding 2.5-3 cm in diameter are the hallmark finding 2
- Air-fluid levels visible on upright films represent the classic radiographic sign 1, 2
- Relative or complete absence of colonic gas helps distinguish small-bowel from large-bowel obstruction 1
- Serial examinations showing persistent dilated loops with air-fluid levels increase diagnostic confidence 1
High-Grade Obstruction Indicators
Two specific findings on upright films are most predictive of high-grade or complete obstruction:
- Air-fluid levels of differential height within the same small-bowel loop (p ≤ 0.0003) 3
- Mean air-fluid level width ≥25 mm on upright films (p ≤ 0.0003) 3
When both critical findings are present, the obstruction is likely high-grade or complete; when both are absent, obstruction is likely low-grade or nonexistent. 3
Critical Limitations of Abdominal Radiographs
Despite these characteristic findings, abdominal series have significant limitations:
- Diagnostic accuracy is only 30-90% with sensitivity and specificity of 60-70% 4, 2
- Radiographs are not definitive in many patients and may prolong evaluation 1
- Plain films cannot identify the transition point, cause of obstruction, or complications such as ischemia or closed-loop obstruction 2
Common Pitfalls to Avoid
- Relying solely on plain radiographs can delay diagnosis and appropriate treatment 4
- Low-grade or intermittent obstruction may appear unremarkable on standard films, with sensitivity dropping to only 48-50% 1, 5, 4
- High obstructions, strangulation, partial obstruction, and fluid-filled proximal bowel may produce equivocal or unrevealing plain films 6
When to Proceed Directly to CT
The American College of Radiology recommends CT abdomen/pelvis with IV contrast as the gold standard, with diagnostic accuracy exceeding 90%. 5, 4, 2
CT should be obtained when:
- High clinical suspicion exists for small-bowel obstruction 2
- Abdominal radiographs are equivocal or inconclusive 1, 4
- You need to identify the transition point, etiology, or complications 5, 2
- IV contrast is essential to assess for bowel ischemia, which plain films cannot detect 4, 2
- No oral contrast is needed as fluid-filled dilated bowel provides natural contrast 2
Multiplanar CT reconstructions significantly increase accuracy in localizing the transition zone and should be utilized 5, 4, 2