What lab tests are recommended for a patient presenting with symptoms of Irritable Bowel Syndrome (IBS)?

Laboratory Screen for IBS

For patients presenting with IBS symptoms, perform celiac serology (IgA tissue transglutaminase with total IgA) and stool testing for Giardia as the two strongest evidence-based tests, then add fecal calprotectin or fecal lactoferrin to screen for inflammatory bowel disease. 1

Core Recommended Tests (Strong Evidence)

Essential Tests for All Patients

• Celiac disease screening: IgA tissue transglutaminase (IgA-tTG) PLUS total IgA level to detect IgA deficiency (strong recommendation, moderate quality evidence) 1, 2, 3, 4

  • If IgA deficient, use IgG-based testing (IgG-deamidated gliadin peptide or IgG-tTG) 1
  • Sensitivity >90% for detecting celiac disease, which commonly mimics IBS 2

• Stool testing for Giardia (strong recommendation, high quality evidence) 1, 2

  • Giardia is the most common parasitic cause of chronic diarrhea that presents as IBS 2

Inflammatory Bowel Disease Screening

• Fecal calprotectin OR fecal lactoferrin (conditional recommendation, low quality evidence) 1, 2, 3, 4
  • Particularly important in patients under age 45 with diarrhea 2, 3
  • More sensitive than blood inflammatory markers for detecting IBD 1

Additional Tests Based on Clinical Context

Complete Blood Count

• CBC to screen for anemia and inflammatory processes 2, 3, 5
  • Anemia is an alarm feature requiring more extensive workup 3

Tests to Consider (Conditional Recommendations)

• Bile acid diarrhea testing in patients with IBS-D who don't respond to initial therapy (conditional recommendation, low quality evidence) 1

  • Options include: 48-hour stool bile acid collection, serum FGF-19, or SeHCAT scanning (if available) 1, 2
  • Since these tests are often unavailable, empiric trial of bile acid binders is reasonable 1

• Lactose breath testing for patients consuming >0.5 pint (280 ml) of milk daily, especially those from high-risk ethnic groups 1, 2, 3

  • Patient-reported lactose intolerance correlates poorly with objective evidence 1

Tests NOT Recommended

Avoid These Blood Tests for IBD Screening

• Do NOT use ESR or CRP alone to screen for IBD (conditional recommendation against, low quality evidence) 1, 2
  • Approximately 20% of patients with active Crohn's disease have normal CRP 2
  • These have lower sensitivity than fecal markers for detecting IBD 1

Other Tests to Avoid

• Do NOT test for ova and parasites (other than Giardia) unless travel history to or recent immigration from high-risk areas 1, 2

• Do NOT use serologic tests for IBS diagnosis (no recommendation due to knowledge gap) 1

  • Tests for anti-cytolethal distending toxin B and anti-vinculin antibodies have low sensitivity (20-40%) despite high specificity 1

• Do NOT perform ultrasound as it detects incidental asymptomatic findings (gallstones, fibroids) in 8% of patients, leading to inappropriate surgery 1, 2

Age-Specific Considerations

Patients Under 45 Years

• Young patients with typical IBS symptoms and no alarm features can be diagnosed with the minimal testing panel above 2
• No colonoscopy needed if no alarm features present 2

Patients Over 50 Years or High-Risk Features

• Colonoscopy indicated for patients ≥50 years at symptom onset OR positive family history of colorectal cancer, regardless of symptom pattern 2, 3
• Stool occult blood testing recommended 2, 3

Critical Alarm Features Requiring Extensive Workup

If ANY of these are present, perform comprehensive evaluation beyond the basic IBS screen: 3

• Unintentional weight loss
• Rectal bleeding or positive fecal occult blood
• Fever
• Anemia on CBC
• Elevated fecal calprotectin
• Nocturnal diarrhea
• Age >50 years at symptom onset
• Family history of IBD or colorectal cancer
• Acute onset in previously well patient

Common Pitfalls to Avoid

• Don't rely on patient-reported food intolerances without objective testing—this leads to unnecessary dietary restrictions 2
• Don't skip celiac testing even if symptoms seem typical for IBS—celiac disease is common and treatable 1, 2
• Don't order colonoscopy reflexively in young patients without alarm features—it's not cost-effective 2
• Don't assume normal CRP/ESR rules out IBD—use fecal calprotectin instead 1, 2