Laboratory Evaluation for IBS-D
For patients with suspected IBS-D, order celiac serology (IgA tissue transglutaminase with total IgA level), fecal calprotectin, stool testing for Giardia, and complete blood count as your initial laboratory panel. 1, 2
Core Recommended Tests (Strong Evidence)
Celiac Disease Screening
- IgA tissue transglutaminase (IgA-tTG) with total IgA level is mandatory for all patients with chronic diarrhea and abdominal pain, as celiac disease is a major cause of IBS-D symptoms with test sensitivity >90%. 1, 2
- If IgA deficiency is detected, use IgG-based testing (IgG-deamidated gliadin peptide or IgG-tTG) as the alternative. 2
Stool Testing
- Stool testing for Giardia antigen is strongly recommended, as Giardia is a common parasitic cause of chronic diarrhea that mimics IBS-D. 1, 2
- Fecal calprotectin should be ordered to screen for inflammatory bowel disease, particularly in patients under age 45 with diarrhea. 1, 2, 3
Basic Blood Work
- Complete blood count (CBC) to screen for anemia, which may indicate organic disease requiring colonoscopy. 2, 5
Tests with Conditional Recommendations
Inflammatory Markers (Limited Value)
- Do NOT routinely order CRP or ESR to screen for IBD, as these have poor diagnostic accuracy for distinguishing IBS-D from inflammatory bowel disease. 1, 2
- Important caveat: Approximately 20% of patients with active Crohn's disease have normal CRP levels, so normal inflammatory markers do not exclude IBD. 2
Bile Acid Diarrhea Testing
- Consider SeHCAT scanning or serum 7α-hydroxy-4-cholesten-3-one (C4) in patients with IBS-D who fail initial therapy, as bile acid malabsorption may be present in a subset of patients. 1, 2
- Note: SeHCAT is not available in the United States; serum C4 or FGF19 are alternatives. 1
Lactose Intolerance
- Lactose breath testing should be considered if the patient regularly consumes >0.5 pint (280 ml) of milk daily, especially in high-risk ethnic groups. 2
Tests NOT Recommended
Avoid These Tests
- Do NOT order ova and parasite testing unless there is specific travel history to or recent immigration from high-risk endemic areas. 2
- Do NOT order serologic antibody tests for IBS diagnosis (anti-CdtB and anti-vinculin antibodies), as these have insufficient diagnostic accuracy with sensitivity only 20-40% and are not useful for routine clinical practice. 1
- Do NOT order ESR or CRP alone as screening tests for organic disease. 1, 2
Age-Based Colonoscopy Considerations
- Patients under age 45-50 with typical IBS-D symptoms and no alarm features do not require colonoscopy if the above laboratory tests are negative. 2, 5
- Patients over age 50 should undergo colonoscopy regardless of laboratory results due to colorectal cancer screening recommendations. 2, 5
Alarm Features Requiring Colonoscopy
Proceed directly to colonoscopy if any of the following are present, regardless of laboratory results: 5
- Unintentional weight loss
- Blood in stools or positive fecal occult blood
- Anemia (particularly iron deficiency)
- Fever
- Nocturnal symptoms that wake the patient from sleep
- Abnormal physical examination findings
- Family history of colorectal cancer or inflammatory bowel disease
Common Pitfalls to Avoid
- Do not delay diagnosis by ordering extensive testing in young patients (<45 years) with typical symptoms and negative basic laboratory screening—IBS-D is a positive symptom-based diagnosis using Rome criteria, not a diagnosis of exclusion. 2
- Do not rely on normal CRP/ESR to definitively exclude IBD, as inflammatory markers have poor sensitivity. 1, 2
- Do not order ultrasound, as it frequently detects incidental asymptomatic findings unrelated to IBS-D symptoms. 2
- Do not perform hydrogen breath testing for small intestinal bacterial overgrowth in typical IBS-D, as this is not recommended by current guidelines. 2