What labs are drawn to diagnose Crohn's disease?

Laboratory Testing for Diagnosing Crohn's Disease

At initial diagnosis, draw a complete blood count, C-reactive protein, albumin, liver function tests, iron studies, renal function, vitamin B12, and fecal calprotectin to establish baseline values and support the diagnostic workup, though these labs alone cannot diagnose Crohn's disease—ileocolonoscopy with biopsies remains the diagnostic gold standard. 1, 2

Essential Baseline Laboratory Panel

The following labs should be obtained in all patients with suspected Crohn's disease at initial evaluation:

• Complete blood count (CBC): Assesses for anemia (from chronic inflammation, blood loss, or malabsorption), leukocytosis (indicating active inflammation), and thrombocytosis (correlates with disease activity) 1, 2
• C-reactive protein (CRP): Elevated in approximately 80% of patients with active Crohn's disease, though notably 20% may have normal CRP despite active disease 1, 3
• Albumin: Low levels indicate malnutrition, protein-losing enteropathy, or severe inflammation 1, 2
• Liver function tests: Screens for hepatobiliary complications and establishes baseline before initiating immunosuppressive therapy 1, 2
• Iron studies: Identifies iron deficiency anemia, common in Crohn's disease from chronic blood loss and malabsorption 1, 2
• Renal function: Establishes baseline and screens for complications like nephrolithiasis 1, 2
• Vitamin B12: Particularly important as terminal ileal disease impairs B12 absorption 1, 2

Fecal Biomarkers for Diagnostic Support

Fecal calprotectin is the single most useful stool test to differentiate inflammatory bowel disease from functional disorders, with 93% sensitivity and 96% specificity at a threshold of 100 μg/g. 1, 2, 4

• Fecal calprotectin or lactoferrin should be measured in all patients at diagnosis 1
• These markers correlate better with endoscopic inflammation than CRP or white blood cell count 1
• A threshold of 100 μg/g provides optimal diagnostic precision compared to 50 μg/g 1
• Critical caveat: Fecal calprotectin may be normal in patients with isolated ileal Crohn's disease despite active endoscopic inflammation 1

Inflammatory Markers and Their Limitations

CRP is useful but imperfect—it correlates with disease activity and has prognostic value, but normal CRP does not exclude active Crohn's disease. 1

• CRP elevation correlates with response to anti-TNF therapy and predicts mucosal healing 1
• Erythrocyte sedimentation rate (ESR) is less reliable than CRP and does not closely reflect disease activity in individual patients 5
• Neither CRP nor ESR alone can distinguish Crohn's disease from other inflammatory conditions 1, 3

Mandatory Infectious Exclusion

Before confirming an IBD diagnosis, obtain stool specimens to exclude:

• Common bacterial pathogens (Salmonella, Shigella, Campylobacter, E. coli O157:H7) 2, 6
• C. difficile toxin testing is mandatory 2, 6
• Consider testing for parasites and other pathogens based on clinical context 2

Why Labs Alone Cannot Diagnose Crohn's Disease

No laboratory test is diagnostic of Crohn's disease—the diagnosis requires ileocolonoscopy with biopsies demonstrating characteristic histologic features (focal, asymmetric, transmural inflammation, or granulomas) combined with endoscopic findings (skip lesions, strictures, fistulas). 1, 2, 7

Laboratory parameters serve three critical but limited roles:

1. Supporting the diagnosis by demonstrating inflammation and excluding alternative diagnoses 1, 2
2. Assessing nutritional status and complications that require intervention 1, 2, 3
3. Establishing baseline values for monitoring disease activity and treatment response 1

Additional Testing Considerations

• Serological markers (pANCA, ASCA) have limited diagnostic accuracy and are not routinely recommended 6
• Vitamin D levels should be checked given high prevalence of deficiency and bone health implications 1, 3
• Folate levels may be assessed, particularly in patients with extensive small bowel disease or on methotrexate 3

Common Pitfalls to Avoid

• Do not rely on normal CRP or fecal calprotectin to exclude Crohn's disease, especially in isolated ileal disease 1
• Do not use laboratory values alone to make treatment decisions—endoscopic assessment remains the gold standard for evaluating mucosal inflammation 1
• Do not forget to exclude C. difficile infection before attributing symptoms to IBD flare 2, 6
• Do not skip vitamin B12 testing, as deficiency is common and often clinically silent until severe 1