What laboratory tests are available for diagnosing Crohn’s disease and ulcerative colitis?

Laboratory Tests for Crohn's Disease and Ulcerative Colitis

No single laboratory test can diagnose Crohn's disease or ulcerative colitis—diagnosis requires a combination of clinical, biochemical, endoscopic, imaging, and histological investigations. 1

Essential Laboratory Tests at Initial Evaluation

Blood Tests

• Complete blood count (CBC) to detect anemia (hemoglobin <13 g/dL for men, <12 g/dL for women), which affects quality of life and occurs frequently in IBD 1, 2
• C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) as inflammatory markers, though approximately 20% of patients with active Crohn's disease may have normal CRP levels 2, 3
• Serum albumin to assess nutritional status and disease severity 4
• Liver function tests as part of baseline assessment 2
• Renal function tests for baseline evaluation 4
• Vitamin B12 levels, particularly important in Crohn's disease with ileal involvement 4

Iron Status Assessment

The interpretation of iron studies depends critically on inflammation status:

• Without active inflammation: Serum ferritin <30 μg/L indicates iron deficiency 1
• With active inflammation: Serum ferritin up to 100 μg/L may still indicate iron deficiency 1
• Anemia of chronic disease: Serum ferritin >100 μg/L with transferrin saturation <20% in the presence of inflammation 1, 2
• Mixed picture: Ferritin 30-100 μg/L suggests combined iron deficiency and anemia of chronic disease 1

Stool Tests

• Fecal calprotectin is highly accurate with 93% sensitivity and 96% specificity for diagnosing IBD in adults, with optimal threshold of 100 μg/g 4, 2
• Stool cultures and C. difficile toxin testing are mandatory to exclude infectious causes before confirming IBD diagnosis—loose stools for more than 6 weeks usually discriminate IBD from infectious diarrhea 1, 4, 2
• Ova, cysts, and parasites if travel history or other risk factors are present 1

Serological Markers: Limited Utility

Serological markers have limited accuracy and should not be used as primary diagnostic tools:

• pANCA and ASCA antibodies may support diagnosis but are ineffective at differentiating colonic Crohn's disease from ulcerative colitis 1, 2
• Antiglycan and antimicrobial antibodies (anti-OmpC, CBir1) provide minimal additional diagnostic value 1, 2
• Genetic testing for IBD-associated SNPs is not recommended for diagnosis despite identification of over 250 variants 1, 2

Critical Diagnostic Pitfalls to Avoid

• Never rely on laboratory tests alone—they must be interpreted alongside endoscopic findings, imaging, and histology 2, 5
• Normal CRP does not exclude active disease—up to 20% of patients with active Crohn's disease have normal CRP 2
• Always exclude infectious causes first before making an IBD diagnosis to avoid inappropriate treatment 1, 2
• Interpret iron studies in the context of inflammation—failure to do so leads to missed iron deficiency diagnosis 1, 2
• Do not use serological markers alone to differentiate between Crohn's disease and ulcerative colitis 1, 2

The Diagnostic Algorithm

Laboratory tests serve as supportive evidence within a comprehensive diagnostic pathway:

1. Initial blood work (CBC, CRP/ESR, albumin, liver/renal function, iron studies, B12) establishes baseline and identifies complications 4, 2
2. Stool testing excludes infectious causes and provides fecal calprotectin for IBD likelihood 1, 2
3. Ileocolonoscopy with biopsies from inflamed and uninflamed segments remains the gold standard for diagnosis 1, 4
4. Cross-sectional imaging (MR enterography preferred) evaluates extent and complications beyond endoscopic reach 4, 3
5. Histology confirms diagnosis, though it is often "compatible with" rather than "diagnostic of" IBD 5

Laboratory tests cannot establish or rule out IBD independently—they function as one component of a multimodal diagnostic approach that prioritizes endoscopic visualization and histological confirmation. 1, 2, 5