What X-ray (radiograph) do I order for constipation?

Plain Abdominal X-Ray (KUB)

For routine constipation evaluation, order a plain abdominal radiograph (KUB) only when you need to exclude bowel obstruction or assess fecal impaction severity—not for routine diagnosis, as clinical assessment alone is sufficient and more reliable. 1

When to Order Imaging

Do NOT routinely order imaging for:

• Uncomplicated constipation diagnosis - Clinical assessment (history, digital rectal exam with pelvic floor assessment during simulated evacuation) is sufficient and more accurate than radiography 1, 2
• Initial evaluation in patients without alarm features - Plain films have limited diagnostic utility and do not significantly change management 1, 3

DO order plain abdominal X-ray (KUB) when:

• Suspected bowel obstruction - To exclude mechanical obstruction in patients with severe pain, vomiting, inability to pass flatus, or acute symptom onset 1
• Assessing extent of fecal impaction - When digital rectal exam suggests severe impaction and you need to visualize the full extent before disimpaction 1
• Differentiating overflow diarrhea from true diarrhea - When diarrhea accompanies constipation symptoms, suggesting impaction with overflow 1

Critical Limitations of Plain Abdominal X-Ray

Plain radiography has poor diagnostic accuracy for constipation:

• Sensitivity for bowel obstruction is only 74-84% with specificity of 50-72% 1, 4
• In one emergency department study, 55% of patients with no/mild stool burden on X-ray were still diagnosed with constipation, and 42% with moderate/large stool burden received no constipation treatment 3
• Fecal loading on radiography does not exclude more serious diagnoses 3

Alternative Imaging When Plain X-Ray is Inadequate

For suspected bowel obstruction:

• CT scan is superior - Sensitivity 93-96% and specificity 93-100% compared to plain films 4
• Order CT when clinical suspicion is high despite negative or equivocal plain films 1, 4

For structural evaluation (alarm features present):

Order these instead of plain X-ray when you need to exclude structural causes:

• Colonoscopy - First-line for patients >50 years without prior screening, or any age with blood in stool, anemia, weight loss, or abrupt symptom onset 1, 2
• CT colonography - Alternative when colonoscopy contraindicated 1, 2
• Flexible sigmoidoscopy with barium enema - Effective combination for structural imaging 1, 2

For functional assessment (refractory cases):

• Colonic transit study with radiopaque markers - For suspected slow transit constipation after failed initial treatment 1, 2
• Defecography (fluoroscopic or MR) - For suspected defecatory disorders when symptoms persist despite normal anorectal testing 1, 2

Alarm Features Requiring Structural Imaging (Not Plain X-Ray)

Proceed directly to colonoscopy or CT (not plain films) when:

• Blood in stool 1, 2
• Unintentional weight loss 1, 2
• Anemia 1, 2
• Age >50 without prior colorectal cancer screening 1, 2
• Abrupt onset of constipation 2

Common Pitfalls to Avoid

• Do not rely on plain X-ray for diagnosis - It frequently leads to treatment that contradicts radiographic findings 3
• Do not skip digital rectal examination - This provides more clinically useful information than plain radiography and must include assessment of pelvic floor motion during simulated defecation 1, 2
• Do not order metabolic tests routinely - Unless clinical features suggest hypercalcemia, hypothyroidism, or diabetes, limit testing to complete blood count only 1, 2
• Do not use plain films to guide treatment decisions - Clinical assessment is more predictive of treatment response 3

Practical Algorithm

1. Perform thorough digital rectal exam with pelvic floor assessment during simulated evacuation 1, 2
2. If alarm features present → Order colonoscopy or CT (not plain X-ray) 1, 2
3. If severe pain/obstruction suspected → Order plain X-ray to exclude obstruction; if equivocal, proceed to CT 1, 4
4. If impaction suspected on exam → Plain X-ray may help visualize extent before disimpaction 1
5. For uncomplicated constipation → No imaging needed; begin empiric treatment with osmotic or stimulant laxatives 1