What laboratory tests should be obtained for a patient presenting with irritable bowel syndrome (IBS)?

Laboratory Testing for IBS Patients

For patients with typical IBS symptoms and no alarm features, minimal laboratory testing is required: obtain only a complete blood count (CBC) and C-reactive protein (CRP) or ESR, with consideration for celiac serology. 1

Initial Screening Tests (All Patients)

The following basic tests should be obtained in all patients with suspected IBS:

• Complete blood count (CBC) to screen for anemia and exclude inflammatory processes 2, 1
• C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) to assess for occult inflammation 2, 1
• Celiac serology (anti-endomysial antibodies) should be checked in all patients, as celiac disease may occur more frequently in IBS populations 2, 1
• Stool occult blood testing (Hemoccult) for screening purposes 2, 1

Additional Testing Based on Specific Clinical Features

For Diarrhea-Predominant IBS (IBS-D):

• Fecal calprotectin in patients under age 45 with diarrhea to exclude inflammatory bowel disease 1
• Lactose breath testing for patients who regularly consume more than 0.5 pint (280 ml) of milk or equivalent dairy products daily, especially those from ethnic groups with high lactose malabsorption rates 2, 1
• Stool examination for ova, cysts, and parasites based on geographic area and relevant clinical features 2
• SeHCAT scanning or serum 7α-hydroxy-4-cholesten-3-one to exclude bile acid diarrhea in refractory cases 1

For Patients with High Stool Weight (>200 g daily):

• Laxative screen, which is positive in approximately 15-26% of such cases 2

Age and Risk-Based Testing:

• Colonoscopy is recommended for patients over age 50 years or those with a positive family history of colorectal cancer, regardless of symptom pattern 2, 1
• Serum chemistries and albumin can be ordered based on symptom pattern and clinical features 2

Tests That Should NOT Be Routinely Ordered

The following tests have poor diagnostic yield and should be avoided in typical IBS without alarm features:

• Colonoscopy or sigmoidoscopy in young patients (<45 years) with typical symptoms and no alarm features 1
• Abdominal ultrasound, as it detects incidental asymptomatic findings (gallstones, fibroids) in 8% of cases that may lead to inappropriate surgery 2, 1
• Hydrogen breath testing for small intestinal bacterial overgrowth 1
• Testing for exocrine pancreatic insufficiency 1
• Thyroid function tests (despite a 6% yield in one large study, this is not routinely recommended) 2

Critical Alarm Features Requiring Extensive Workup

If any of the following are present, proceed with comprehensive diagnostic evaluation rather than minimal testing:

• Fever, significant weight loss, or rectal bleeding 2
• Anemia or abnormal physical findings on examination 2
• Nocturnal symptoms or acute symptom onset in previously well-controlled disease 2
• Age over 50 years at symptom onset 2
• Family history of inflammatory bowel disease or colorectal cancer 2
• Elevated inflammatory markers (CRP, ESR) or fecal calprotectin 2

Important Clinical Pitfalls

Do not rely on patient-reported food intolerances without objective testing, as this correlates poorly with actual malabsorption and leads to unnecessary dietary restrictions 2, 1. Avoid performing extensive testing in young patients with typical Rome criteria symptoms and no alarm features, as the probability of organic disease is very low and this approach is not cost-effective 3, 4. Remember that fecal calprotectin values between 50-250 μg/g can be challenging to interpret, as mild elevation may represent non-specific low-grade inflammation rather than true IBD 2.