What blood work is required for diagnosing Crohn's (Chronic Regional Ileitis) disease?

Blood Work Required for Diagnosing Crohn's Disease

For suspected Crohn's disease, routine blood tests should include complete blood count, C-reactive protein, erythrocyte sedimentation rate, liver profile, albumin, iron studies, renal function, vitamin B12, and fecal calprotectin as the minimum laboratory workup. 1, 2

Essential Laboratory Tests

First-Line Blood Tests

• Complete blood count (CBC)

  • Helps detect anemia (Hb <13 g/dL for men, <12 g/dL for women)
  • May show thrombocytosis in chronic inflammation
  • Can reveal leukocytosis which might indicate infection

• Inflammatory markers

  • C-reactive protein (CRP)

    • Important diagnostic and prognostic value
    • Note: approximately 20% of active Crohn's patients may have normal CRP 1

  • Erythrocyte sedimentation rate (ESR)

    • Helps assess chronic inflammation

• Biochemical parameters

  • Liver function tests (ALT, AST, ALP, GGT, bilirubin)
  • Albumin (low levels indicate malnutrition or severe disease)
  • Renal function tests (creatinine, urea)

• Iron studies

  • Serum ferritin
  • Transferrin saturation
  • Important for diagnosing iron deficiency anemia 1
  • Diagnostic criteria depend on inflammation level:
    • Without inflammation: serum ferritin <30 μg/L indicates iron deficiency
    • With inflammation: serum ferritin up to 100 μg/L may still indicate iron deficiency 1

Stool Tests

• Fecal calprotectin

  • Critical biomarker with high sensitivity (93%) and specificity (96%) 1, 2
  • Values <50 μg/g make IBD unlikely
  • Values >150 μg/g strongly suggest active inflammation
  • More accurate at cut-off of 100 μg/g compared to 50 μg/g 1

• Fecal lactoferrin

  • Alternative stool biomarker that helps differentiate IBD from IBS 1

• Stool cultures

  • To exclude infectious causes
  • Specifically test for Clostridium difficile toxin
  • Consider tests for ova, parasites, and other pathogens based on history 1, 2

Second-Line Laboratory Tests

• Vitamin B12 levels

  • Particularly important in patients with ileal disease or resection 1

• Folate levels

  • To assess nutritional status and detect deficiencies

• Vitamin D level

  • Often deficient in Crohn's disease patients 2

• More extensive workup (when diagnosis remains unclear)

  • Percentage of hypochromic red cells
  • Reticulocyte count and reticulocyte hemoglobin
  • Haptoglobin
  • Lactate dehydrogenase
  • Soluble transferrin receptor 1

Interpretation of Results

• Anemia evaluation

  • If hemoglobin is below normal (men <13 g/dL, women <12 g/dL), initiate anemia workup 1
  • MCV and RDW help differentiate types of anemia:
    • Microcytic (low MCV): suggests iron deficiency
    • Macrocytic (high MCV): suggests B12 or folate deficiency
    • Normocytic: may indicate anemia of chronic disease 1

• CRP and ESR interpretation

  • Elevated CRP with normal ESR: acute inflammation
  • Normal CRP with elevated ESR: chronic inflammation
  • Both elevated: active inflammation 2

Clinical Pearls and Pitfalls

• Important caveat: No single test can definitively diagnose Crohn's disease; diagnosis requires a combination of clinical, laboratory, endoscopic, and histological findings 1, 2

• Potential pitfall: Relying solely on CRP for diagnosis, as approximately 20% of patients with active Crohn's disease may have normal CRP levels 1

• Key consideration: Laboratory tests should be conducted at diagnosis to establish baseline values for future comparison 1

• Diagnostic challenge: Differentiating Crohn's disease from other conditions like intestinal tuberculosis, which may require additional specialized testing 3

• Follow-up monitoring: After diagnosis, regular monitoring of disease activity using calprotectin (every 3-6 months) and monitoring for nutritional deficiencies are essential 2

While these laboratory tests are crucial for diagnosis, they must be interpreted alongside endoscopic findings (ileocolonoscopy with biopsies), cross-sectional imaging (CT or MR enterography), and clinical symptoms to establish a definitive diagnosis of Crohn's disease 1, 2.