What is the preferred test to rule out constipation?

Preferred Tests to Rule Out Constipation

Digital rectal examination (DRE) is the preferred initial test to rule out constipation, as it is a safe, simple diagnostic tool that can effectively assess for impacted feces, sphincter tone, and pelvic floor dysfunction. 1, 2

Initial Assessment

History and Physical Examination

• Detailed bowel history including:
  • Date of last defecation
  • Frequency of bowel movements
  • Stool consistency
  • Recent changes in bowel patterns
  • Urge to defecate
  • Sensation of evacuation (complete or incomplete)
  • Presence of blood or mucus
  • Current and previous laxative use
  • Need for digital manipulation 1

Digital Rectal Examination

The DRE should include:

• Assessment of sphincter tone
• Evaluation for tenderness
• Detection of impacted feces
• Identification of masses or obstruction
• Assessment of pelvic floor motion during simulated evacuation 1, 3

A properly performed DRE can yield diagnostic accuracy comparable to specialized physiologic tests, including anorectal manometry 3. However, a normal DRE does not completely exclude defecatory disorders 1.

Laboratory Tests

• Complete blood count (CBC) is the only necessary laboratory test in the absence of other symptoms or signs 1
• Metabolic tests (glucose, calcium, thyroid-stimulating hormone) are not recommended for routine evaluation of chronic constipation unless other clinical features warrant them 1, 2

Imaging Studies

Plain Abdominal Radiography

• Can be used to evaluate fecal load and rule out intestinal obstruction 2, 4
• First-line imaging for constipation and suspected bowel obstruction 2
• Can show the extent of fecal impaction in chronic constipation 4

Colonoscopy

• Should not be performed in patients without alarm features (blood in stools, anemia, weight loss) unless age-appropriate colon cancer screening has not been performed 1, 5
• Indicated when patients have alarm symptoms/signs that suggest an organic cause for constipation, such as:
  • Age > 50 years with new-onset constipation
  • Gastrointestinal bleeding
  • Palpable abdominal or rectal mass
  • Weight loss
  • Anemia
  • Family history of colorectal cancer 5

Advanced Testing (for refractory cases)

If initial management fails after 4-6 weeks, consider:

1. Colonic transit studies - To diagnose slow-transit constipation 1, 2, 6

   • Should be evaluated if anorectal test results do not show a defecatory disorder or if symptoms persist despite treatment 1

2. Anorectal manometry - To identify anal weakness and rectal sensation issues 2, 6

3. Defecography (evacuation proctography) - To diagnose functional disorders of the rectum and anus, such as rectocele, intussusception, and abnormal perineal floor descent 4

4. CT scan with contrast - For suspected obstruction, to identify the transition point and potential causes 2

Common Pitfalls to Avoid

1. Overreliance on imaging: Investigations are not routinely necessary for most cases of constipation 1

2. Premature colonoscopy: Performing colonoscopy without alarm features is not cost-effective and exposes patients to unnecessary risks 1, 5

3. Overlooking medication effects: Failing to review and adjust medications that may cause constipation before further testing 1

4. Incomplete DRE: Performing a cursory examination without assessing pelvic floor motion during simulated evacuation 1, 3

5. Missing defecatory disorders: These are present in approximately two-thirds of adult patients with chronic constipation and require specific testing for diagnosis 6

In summary, the diagnostic approach to constipation should start with a thorough history and digital rectal examination, followed by a CBC. Further testing should be guided by the presence of alarm features and response to initial management.