What are the diagnostic steps to test for Crohn's disease?

Diagnostic Testing for Crohn's Disease

The diagnosis of Crohn's disease requires a combination of clinical, biochemical, stool, endoscopic, cross-sectional imaging, and histological investigations, with ileocolonoscopy with biopsies being the essential first-line test for suspected IBD. 1

Initial Diagnostic Workup

First-Line Testing

1. Ileocolonoscopy with biopsies

   • Essential first diagnostic step for suspected IBD 1
   • Multiple biopsies should be taken from:
     • Inflamed segments
     • Uninflamed segments
     • Every colonic segment including the rectum 1
   • Allows direct visualization of mucosal inflammation and ulceration 1
   • Look for key endoscopic features of Crohn's disease:
     • Discontinuous lesions
     • Presence of strictures and fistulae
     • Perianal involvement 1

2. Laboratory investigations

   • Complete blood count
   • Liver profile
   • Albumin
   • Iron studies
   • Renal function
   • Vitamin B12 1
   • C-reactive protein (CRP)
     • Important diagnostic and prognostic marker
     • Note that approximately 20% of patients with active CD may have normal CRP levels 1, 2

3. Stool tests

   • Fecal calprotectin
     • High sensitivity (93%) and specificity (96%) to diagnose IBD in adults 1
     • Helps differentiate IBD from IBS or functional disease 1, 2
     • More accurate with a cut-off of 100 μg/g compared to 50 μg/g 1
   • Stool analysis and culture
   • C. difficile toxin testing 1
   • Tests for ova, cysts, and parasites (especially with travel history) 1

Second-Line Testing

1. Cross-sectional imaging

   • Required in all patients at diagnosis to assess extent and severity of disease 1
   • MR enterography (preferred) or CT enterography
     • Evaluates small bowel beyond the reach of colonoscopy 1
     • Detects transmural disease with normal overlying mucosa 1
     • Requires proper bowel distention with large volumes (1300-1800 cc) of neutral contrast 1
   • Small bowel ultrasound is an alternative option 1

2. Additional endoscopic procedures

   • Upper GI endoscopy with biopsies
     • Particularly useful in pediatric patients and adults with upper GI symptoms 1, 3
   • Small bowel capsule endoscopy (SBCE)
     • Recommended for patients with high clinical suspicion of CD but inconclusive ileocolonoscopy and imaging 1
   • Enteroscopy (single or double-balloon)
     • When biopsy of abnormal areas is beyond the reach of standard endoscopes 1

Diagnostic Features to Look For

Endoscopic Features

• CD characteristics: Discontinuous lesions, skip areas, cobblestone appearance, deep ulcerations, strictures, fistulae 1
• Precise description of lesions including type, location, depth, and extent is essential 1
• Scoring systems like CDEIS (Crohn's Disease Endoscopic Index of Severity) or SES-CD (Simple Endoscopic Score for Crohn's Disease) can be used to assess severity 1

Histological Features

• Key CD findings: Granulomas, focal crypt architectural abnormalities, focal/patchy chronic inflammation, mucin preservation at active sites 1
• Multiple features should be present for diagnosis, with granulomas being most specific 1
• Biopsies from both inflamed and uninflamed areas are crucial to document skip lesions 1

Common Pitfalls and Caveats

• No single test is diagnostic for Crohn's disease; a comprehensive approach is required 1
• Disease location varies: Small bowel alone (33%), colon alone (>33%), combined small bowel and colon (<33%) 1
• Upper GI involvement may be missed if not specifically investigated 3
• Normal CRP doesn't rule out active disease - approximately 20% of patients with active CD have normal CRP 1
• Differential diagnosis must exclude:
  • Infectious colitis (preserved crypt architecture with acute inflammation) 1
  • Segmental colitis associated with diverticulitis (SCAD) 1
  • Ischemic colitis 1
  • Other causes of granulomatous inflammation 3

Follow-up Testing

• Establish baseline values for all laboratory and inflammatory markers for future comparison 1
• Regular monitoring with fecal calprotectin and CRP to assess disease activity 2
• Periodic endoscopic assessment to evaluate mucosal healing 1
• Post-surgical monitoring using the Rutgeerts score to assess for recurrence at the neoterminal ileum 1, 4

By following this comprehensive diagnostic approach, clinicians can accurately diagnose Crohn's disease, determine disease extent and severity, and establish a baseline for monitoring treatment response and disease progression.