Is There a Specific Blood Test for Diagnosing IBS?
No, there is no specific blood test that can diagnose IBS—the diagnosis is made clinically using symptom-based criteria (Rome criteria), and blood tests are only used to exclude other diseases that mimic IBS symptoms. 1
How IBS Should Be Diagnosed
IBS diagnosis requires a positive symptom-based approach using the Rome criteria: abdominal pain for at least 12 weeks in the preceding 12 months with at least two of three features: pain relieved by defecation, onset associated with change in stool frequency, or onset associated with change in stool form. 1 The diagnosis always presumes the absence of structural or biochemical explanations for symptoms. 1
Blood Tests to Rule Out Other Conditions (Not to Diagnose IBS)
While no blood test diagnoses IBS, certain tests help exclude organic diseases:
Recommended Blood Tests
Complete blood count (CBC) should be performed in all patients with suspected IBS to screen for anemia and other hematologic abnormalities. 1
Celiac serology (IgA tissue transglutaminase with total IgA) is a strong recommendation with moderate-quality evidence, as celiac disease commonly mimics IBS symptoms with sensitivity >90%. 1, 2 The British Society of Gastroenterology recommends checking anti-endomysial antibodies in all patients with IBS symptoms. 1
C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) should be performed according to European guidelines 1, though the American Gastroenterological Association suggests against using these due to low-quality evidence. 1 This represents a divergence in guideline recommendations—European guidelines favor their use while American guidelines are more conservative.
Blood Tests That Are NOT Recommended
There is no commercially available blood test that can diagnose IBS itself. 1 Serologic tests marketed for IBS diagnosis (such as anti-CdtB and anti-vinculin antibodies) are not recommended due to insufficient evidence, with sensitivity <50%, meaning negative tests cannot rule out IBS. 1
Stool Tests (Also to Exclude Other Conditions)
Fecal calprotectin should be tested in patients with diarrhea under age 45 to exclude inflammatory bowel disease. 1, 2 This is a strong recommendation with high-quality evidence.
Stool testing for Giardia is a strong recommendation with high-quality evidence, as Giardia is a common parasitic cause of chronic diarrhea. 1
Stool hemoccult testing is recommended for screening purposes. 1
Age-Specific Approach
Young patients (<45 years) with typical IBS symptoms and no alarm features may be safely given a working diagnosis without extensive testing. 1 This avoids unnecessary procedures and reduces healthcare costs while maintaining diagnostic safety.
Patients over 50 years old or with a family history of colorectal cancer should undergo colonoscopy regardless of symptom pattern. 1 This is critical because age >50 is itself an alarm feature requiring structural evaluation.
Common Pitfalls to Avoid
Do not order colonoscopy in young patients with typical IBS symptoms and no alarm features—this is not cost-effective and exposes patients to unnecessary risk. 1
Do not rely on ultrasound, as it often detects incidental findings unrelated to symptoms. 1
Do not perform hydrogen breath testing for small intestinal bacterial overgrowth in patients with typical IBS symptoms—this is not recommended. 1
Approximately 20% of patients with active Crohn's disease may have normal CRP levels, so a normal inflammatory marker does not completely exclude inflammatory bowel disease. 3 This highlights the limitation of relying solely on CRP/ESR.