Laboratory Tests for Diagnosing IBD and Crohn's Disease
There is no single laboratory test that can definitively diagnose Inflammatory Bowel Disease (IBD) or Crohn's disease; diagnosis requires a combination of clinical, biochemical, stool, endoscopic, radiological, and histological investigations. 1
Initial Laboratory Investigations
• Complete blood count (CBC) should be performed to check for anemia, which is a common complication of IBD 1
• C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) are important inflammatory markers that can indicate active disease, though approximately 20% of patients with active Crohn's disease may have normal CRP levels 1
• Liver function tests and electrolytes (U&Es) should be included in the initial workup 1
• Stool tests for infectious pathogens including Clostridium difficile toxin are essential to exclude infectious causes of diarrhea that can mimic IBD 1
• Fecal calprotectin and fecal lactoferrin are highly sensitive non-invasive biomarkers that can help differentiate IBD from irritable bowel syndrome (IBS) with sensitivity of 93% and specificity of 96% in adults 1, 2
Serological Markers
• pANCA (perinuclear anti-neutrophil cytoplasmic antibodies) and ASCAs (anti-Saccharomyces cerevisiae antibodies) may be used to support a diagnosis, though their accuracy is limited and they are ineffective at differentiating colonic Crohn's from ulcerative colitis 1
• Antiglycan and antimicrobial antibodies (such as anti-OmpC and CBir1) provide minimal additional diagnostic value 1
• Genetic testing for IBD-associated susceptibility single nucleotide polymorphisms (SNPs) is not recommended for diagnosis despite identification of over 250 IBD-associated variants 1
Iron Deficiency Assessment
• Serum ferritin interpretation depends on inflammation status:
- In patients without evidence of active disease: serum ferritin <30 μg/L indicates iron deficiency 1
- In patients with inflammation: serum ferritin up to 100 μg/L may still indicate iron deficiency 1
- With biochemical evidence of inflammation: serum ferritin >100 μg/L and transferrin saturation <20% indicates anemia of chronic disease 1
- Ferritin levels between 30-100 μg/L with inflammation likely represent a combination of true iron deficiency and anemia of chronic disease 1
Important Considerations
• Laboratory tests alone cannot diagnose IBD or Crohn's disease - they must be interpreted alongside endoscopic findings, imaging, and histology 1
• Ileocolonoscopy with biopsies remains the gold standard procedure for diagnosing IBD, allowing direct visualization of the mucosa and collection of tissue samples 1
• Cross-sectional imaging (CT enterography, MR enterography, or ultrasound) is necessary to assess the extent and severity of disease, particularly for evaluating small bowel involvement in Crohn's disease 1
• Small bowel capsule endoscopy may be useful in cases where other investigations are inconclusive, but should be preceded by cross-sectional imaging or patency capsule to avoid capsule retention in patients with suspected strictures 1
Common Pitfalls to Avoid
• Relying solely on laboratory markers for diagnosis can lead to misdiagnosis, as no single test is specific or sensitive enough for IBD 1
• Normal CRP does not exclude active disease, as approximately 20% of patients with active Crohn's disease may have normal CRP levels 1
• Failure to exclude infectious causes before making an IBD diagnosis can lead to inappropriate treatment 1
• Interpreting iron studies without considering the impact of inflammation can lead to missed iron deficiency diagnosis 1
• Using serological markers (pANCA, ASCAs) alone to differentiate between Crohn's disease and ulcerative colitis is unreliable 1
In summary, while laboratory tests play an important role in the diagnostic workup of IBD and Crohn's disease, they must be used as part of a comprehensive diagnostic approach that includes endoscopy, imaging, and histopathology to establish a definitive diagnosis 1.