What blood tests are needed to rule out gastrointestinal problems like Irritable Bowel Syndrome (IBS)?

Blood Tests to Rule Out Gastrointestinal Problems Like IBS

For patients with suspected IBS, basic blood tests should include a complete blood count, C-reactive protein or erythrocyte sedimentation rate, and coeliac serology, along with faecal calprotectin in patients under 45 years with diarrhoea to exclude inflammatory bowel disease. 1

Essential Blood Tests for Initial Evaluation

• Complete blood count (CBC) - to assess for anemia which may indicate gastrointestinal bleeding or malabsorption 1
• C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) - inflammatory markers to help exclude inflammatory bowel disease 1
• Coeliac serology (tissue transglutaminase antibodies or endomysial antibodies) - to screen for celiac disease, which can present with similar symptoms to IBS 1
• Serum chemistries and albumin - to assess for electrolyte abnormalities and malnutrition 1

Stool Tests

• Faecal calprotectin - particularly important in patients under 45 years with diarrhoea to exclude inflammatory bowel disease 1
• Stool Hemoccult (occult blood test) - to screen for gastrointestinal bleeding 1
• Stool for ova and parasites - particularly in patients with diarrhoea-predominant symptoms or in endemic areas 1

Additional Tests Based on Symptom Subtype

For Diarrhoea-Predominant Symptoms:

• Lactose/dextrose H2 breath test - to evaluate for lactose intolerance 1
• Serologies for celiac disease - IgA tissue transglutaminase has the best diagnostic accuracy (sensitivity 0.79-0.99, specificity 0.90-0.99) 1
• 23-seleno-25-homotaurocholic acid scanning or serum 7α-hydroxy-4-cholesten-3-one - to exclude bile acid malabsorption, especially in patients with atypical features like nocturnal diarrhoea or history of cholecystectomy 1

For Constipation-Predominant Symptoms:

• Thyroid function tests - to rule out hypothyroidism as a cause of constipation 1

Specialized Tests

• Anti-CdtB and anti-vinculin antibodies - newer biomarkers that may help to "rule in" IBS, particularly post-infectious IBS, though sensitivity is <50% 1

Important Considerations

• IBS is a diagnosis of exclusion that requires ruling out organic diseases that can mimic its symptoms 1, 2
• The diagnostic approach should be guided by the patient's age, predominant symptoms, and presence of any alarm features 1
• Alarm features warranting more extensive evaluation include: rectal bleeding, anemia, nighttime pain, weight loss, family history of inflammatory bowel disease or cancer, and symptom onset after age 50 1, 2
• Colonoscopy is not routinely recommended for IBS diagnosis unless the patient is over 50 years old (for colorectal cancer screening) or has alarm symptoms 1
• In patients with diarrhoea-predominant symptoms, especially those over 50 years with watery diarrhea, colonoscopy with biopsies may be needed to rule out microscopic colitis 1

Diagnostic Algorithm

1. Assess for Rome criteria symptoms and alarm features
2. Order basic blood tests (CBC, CRP/ESR, coeliac serology) and stool tests (faecal calprotectin, occult blood)
3. If basic tests are normal and symptoms meet Rome criteria, consider IBS diagnosis
4. For persistent or concerning symptoms, order additional tests based on predominant symptom pattern
5. Consider specialized tests or referral to gastroenterology if diagnosis remains unclear 1

Remember that IBS is diagnosed based on characteristic symptoms in the absence of structural or biochemical abnormalities, so the primary purpose of these blood tests is to rule out other conditions rather than to confirm IBS 1.