What laboratory tests are drawn for the diagnosis and management of Crohn's disease?

Laboratory Tests for Crohn's Disease Diagnosis and Management

The essential laboratory tests for Crohn's disease include complete blood count, liver profile, albumin, iron studies, renal function, C-reactive protein, and fecal calprotectin or lactoferrin. 1 These tests should be performed in all patients at diagnosis to establish baseline values and monitor disease activity over time.

Initial Diagnostic Laboratory Panel

Blood Tests

• Complete blood count (CBC)

  • Evaluates for anemia (may indicate chronic inflammation or blood loss)
  • Assesses leukocytosis (may indicate inflammation or infection)
  • Checks for thrombocytosis (common in active disease)

• Inflammatory Markers

  • C-reactive protein (CRP)

    • Correlates with clinical severity in Crohn's disease 1
    • Note: Approximately 20% of patients with active Crohn's disease may have normal CRP levels 1

  • Erythrocyte sedimentation rate (ESR)

    • Less specific than CRP but useful for monitoring chronic inflammation

• Biochemical Profile

  • Liver function tests (ALT, AST, ALP, GGT, bilirubin)
  • Albumin (decreased in malnutrition and inflammation)
  • Renal function (creatinine, BUN)
  • Electrolytes

• Nutritional Assessment

  • Iron studies (ferritin, iron, TIBC)
  • Vitamin B12 (especially important in ileal disease)
  • Vitamin D level 2

Stool Tests

• Fecal calprotectin

  • High sensitivity for intestinal inflammation
  • Levels <50 mg/g indicate high sensitivity for ruling out inflammation
  • Levels >250 mg/g indicate higher specificity for active inflammation 2
  • Better correlates with endoscopic inflammation than CRP or white blood cell count 1

• Fecal lactoferrin

  • Alternative marker for intestinal inflammation
  • Can differentiate between clinically active and inactive IBD 1

• Stool cultures and C. difficile testing

  • Essential to exclude infectious causes of symptoms
  • C. difficile testing should be performed with every disease flare 1

Monitoring Laboratory Tests

Frequency of Testing

• At diagnosis: Complete initial panel as baseline 1
• When starting or switching therapy: Repeat complete panel 1
• Routine monitoring: Every 3-12 months as part of global assessment 1
• Post-surgery: 3 months post-surgery, after first endoscopy, and every 3-6 months thereafter 1

Interpretation of Results

• CRP broadly correlates with clinical severity in Crohn's disease (except in proctitis) 1
• CRP >10 mg/L after a year of extensive disease predicts increased risk of surgery 1
• Fecal calprotectin and lactoferrin may be normal in patients with clinically and endoscopically active Crohn's disease, particularly ileal disease 1

Common Pitfalls and Caveats

1. False negatives: Normal CRP doesn't exclude active disease, as approximately 20% of patients with active Crohn's disease may have normal CRP levels 1

2. Ileal disease limitation: Fecal calprotectin and lactoferrin may be normal in patients with isolated ileal Crohn's disease despite active inflammation 1

3. Incomplete assessment: Laboratory tests alone are insufficient for diagnosis - endoscopy with histology remains essential 1

4. Overreliance on single markers: No single laboratory test is sufficiently sensitive or specific; a combination approach is necessary 3, 4

5. Failure to exclude infections: Always test for infectious causes, particularly C. difficile, which can mimic or exacerbate disease flares 1

Laboratory Tests in Different Clinical Scenarios

For Disease Activity Assessment

• CRP, ESR, hemoglobin, hematocrit, and platelet count are the most reliable indicators of Crohn's disease activity 3
• CRP levels correspond closely with clinical and pathological indices of relapse, remission, and response to therapy 5

For Post-Surgical Monitoring

• Complete blood count, liver profile, albumin, iron studies, renal function, vitamin B12, CRP, and fecal calprotectin or lactoferrin should be performed 3 months post-surgery 1

By systematically utilizing these laboratory tests in conjunction with clinical assessment, endoscopy, and imaging, clinicians can effectively diagnose and monitor Crohn's disease activity, optimize treatment decisions, and improve patient outcomes.