What baseline laboratory tests should be ordered for an adult with unexplained abdominal symptoms or suspected colon pathology (e.g., change in bowel habits, abdominal pain, weight loss, anemia, or occult bleeding)?

Laboratory Testing for Abdominal and Colon Abnormalities

Mandatory Baseline Laboratory Panel

All adults presenting with unexplained abdominal symptoms or suspected colon pathology should receive a standardized initial laboratory panel consisting of: complete blood count, fecal calprotectin, celiac serology (IgA tissue transglutaminase with total IgA), stool testing for Giardia, and fecal occult blood test. 1, 2

Core Screening Tests (Order for All Patients)

• Complete blood count (CBC) – Screens for anemia (suggesting occult bleeding or malabsorption) and leukocytosis (indicating infection or inflammation). 1, 2

• Fecal calprotectin – Values <50 µg/g exclude inflammatory bowel disease with 97% specificity; values >250 µg/g strongly suggest IBD and mandate colonoscopy, particularly in patients <45 years with diarrhea. 1, 2, 3

• Celiac serology – IgA tissue transglutaminase (IgA-tTG) with total IgA level has >90% sensitivity for celiac disease, a common cause of chronic diarrhea and IBS-like symptoms. If IgA-deficient, use IgG-based testing (IgG-deamidated gliadin peptide or IgG-tTG). 1, 2

• Stool testing for Giardia antigen – Identifies a treatable parasitic cause of chronic diarrhea with high diagnostic yield. 1, 2

• Fecal occult blood test (FOBT) – Screens for occult gastrointestinal bleeding from colorectal cancer, polyps, or inflammatory lesions. 1, 2, 3, 4

Inflammatory Markers (Conditional Use)

• C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) – May help identify systemic inflammation, but approximately 20% of patients with active Crohn's disease have normal CRP levels, so normal results do not exclude IBD. The American Gastroenterological Association recommends against routine CRP/ESR screening for IBD due to low diagnostic accuracy. 1, 2, 3, 4

• Fecal lactoferrin – Alternative to fecal calprotectin for screening IBD when calprotectin is unavailable. 1, 2

Infection Exclusion Testing

When to Order Stool Cultures

• Stool culture for bacterial pathogens (Salmonella, Shigella, Campylobacter) – Order when diarrhea is accompanied by fever, bloody stools, or acute onset. 3, 5

• Clostridioides difficile testing – Obtain glutamate dehydrogenase antigen and toxin A/B enzyme immunoassays (or nucleic acid amplification) in patients with recent antibiotic use, hospitalization, or healthcare exposure presenting with diarrhea and abdominal pain. 3, 5, 6

• Stool examination for ova and parasites – Reserve for patients with travel history to endemic areas or recent immigration from high-risk regions; routine testing is not recommended. 1, 2

Age-Specific and Alarm Feature-Driven Testing

Patients ≥45–50 Years or With Alarm Features

Any patient ≥45 years at symptom onset OR presenting with alarm features (unintentional weight loss, rectal bleeding, anemia, nocturnal symptoms, fever, family history of IBD or colorectal cancer) requires colonoscopy with biopsies regardless of laboratory results. 1, 2, 4

• During colonoscopy, obtain biopsies from both abnormal-appearing and normal-appearing mucosa; in patients with diarrhea, biopsies are essential to detect microscopic colitis even when mucosa appears normal. 1, 2

Patients <45 Years Without Alarm Features

• Young patients with typical IBS symptoms (Rome criteria: abdominal pain ≥3 days/month for ≥3 months with pain relief after defecation and/or change in stool frequency or form) and negative baseline laboratory panel can receive a positive IBS diagnosis without colonoscopy. 1, 2

• Fecal calprotectin <50 µg/g effectively excludes IBD in this population, making colonoscopy unnecessary. 1, 2

Additional Testing for Persistent Symptoms

When Initial Therapy Fails (3–6 Weeks)

• Bile acid diarrhea assessment – SeHCAT scintigraphy (where available) or serum 7α-hydroxy-4-cholesten-3-one; abnormal bile acid retention is found in 25–33% of patients initially classified as IBS-D. 2

• Lactose breath testing – Consider in patients consuming >0.5 pint (280 mL) of milk daily, especially those from high-risk ethnic groups (Asian, African, Hispanic descent). 1, 2

• Small bowel capsule endoscopy (SBCE) – Reserve for patients with suspected Crohn's disease and normal ileocolonoscopy plus negative cross-sectional imaging, particularly when elevated inflammatory markers or unexplained iron-deficiency anemia persist. 1

Tests NOT Recommended

• Serologic tests for IBS diagnosis – Insufficient evidence; sensitivity <50% means negative tests cannot rule out IBS. 1, 2

• Abdominal ultrasound – Frequently detects incidental asymptomatic findings unrelated to symptoms; not recommended for routine evaluation. 1, 2

• Hydrogen breath testing for small intestinal bacterial overgrowth (SIBO) – Not recommended in patients with typical IBS symptoms. 2

• Testing for exocrine pancreatic insufficiency – Not indicated in typical IBS presentations. 2

Common Pitfalls to Avoid

• Do not rely on normal CRP/ESR to exclude IBD – Up to 20% of active Crohn's disease cases have normal inflammatory markers. 1, 2, 3

• Do not postpone colonoscopy in patients ≥45 years or with alarm features – Age and alarm features mandate endoscopic evaluation regardless of laboratory results. 1, 2, 4

• Do not substitute cross-sectional imaging for colonoscopy – Colonoscopy provides both diagnosis and biopsy capability; CT/MRI cannot exclude mucosal disease. 1, 4

• Do not over-test young patients with typical IBS symptoms – Colonoscopy in patients <45 years without alarm features is not cost-effective and delays appropriate care. 1, 2

• Do not accept patient-reported food intolerances without objective testing – This leads to unnecessary dietary restrictions. 1, 2