What laboratory tests should be ordered for a patient presenting with abdominal bloating, diarrhea, gas, and constipation?

Laboratory Testing for Abdominal Bloating, Diarrhea, Gas, and Constipation

Order initial screening blood tests including complete blood count, celiac serology (IgA tissue transglutaminase with quantitative IgA or IgG-based test if IgA deficient), and stool tests for fecal calprotectin to exclude inflammatory bowel disease, along with testing for Giardia if diarrhea is prominent. 1, 2

Initial Laboratory Workup

The first-line testing strategy should be systematic and based on whether diarrhea or constipation predominates in your patient's presentation:

Blood Tests (All Patients)

• Complete blood count to screen for anemia 2
• Celiac disease serology: IgA tissue transglutaminase (IgA-tTG) is the primary test with >90% sensitivity and specificity 1
  • Critical caveat: Must include either quantitative IgA level OR add IgG-tTG or IgG deaminated gliadin peptides, since IgA deficiency causes false-negative results 1
  • This is a strong recommendation based on moderate-quality evidence that celiac disease is an important treatable cause of these symptoms 1

Stool Tests (Based on Symptom Pattern)

If diarrhea is prominent:

• Fecal calprotectin or fecal lactoferrin to screen for inflammatory bowel disease 1, 2
  • Particularly important if patient is under age 40 with IBS-like symptoms 2
• Giardia testing using antigen detection or PCR (not traditional ova and parasites) 1
  • The AGA specifically recommends against routine ova and parasite testing unless there's travel history to high-risk areas 1

If constipation with bloating predominates:

• Consider fecal immunochemical test (FIT) to exclude occult bleeding, especially if age-appropriate for colorectal cancer screening 2

Second-Line Testing (If Initial Tests Normal and Symptoms Persist)

For Persistent Diarrhea

Bile acid diarrhea testing is crucial and often missed 2, 3:

• SeHCAT testing (if available, primarily in Europe)
• Serum 7α-hydroxy-4-cholesten-3-one (bile acid precursor)
• 48-hour stool bile acid collection
• The British Society of Gastroenterology gives this a strong recommendation to make a positive diagnosis rather than empiric treatment 2

For Microscopic Colitis

• Colonoscopy with biopsies from right and left colon (not rectum) if diarrhea persists 2
  • This is particularly important in older females with chronic diarrhea 4

For Suspected Malabsorption

• Lactose breath testing if lactose intolerance suspected 5, 6
• Comprehensive stool biochemical analysis (osmolality, pH, electrolytes) to differentiate osmotic from secretory diarrhea 3
• Fecal elastase for pancreatic insufficiency if fat malabsorption suspected 3

What NOT to Routinely Order

Avoid these common pitfalls:

• Routine stool ova and parasite testing without travel history - extremely low yield 1
• Extensive infectious workup in chronic symptoms (>4 weeks) without epidemiologic risk factors 3
• Small intestinal bacterial overgrowth (SIBO) breath testing as first-line - its role remains uncertain in IBS 4

Clinical Decision Algorithm

1. Start with blood tests (CBC, celiac serology with IgA status) and fecal calprotectin in all patients 1, 2

2. If tests are negative and symptoms meet Rome criteria, you can confidently diagnose IBS or functional disorder 2, 7

3. If diarrhea persists despite negative initial workup, proceed to bile acid diarrhea testing and consider colonoscopy with biopsies 2

4. If constipation predominates with negative screening, consider rectal examination for dyssynergic defecation before extensive testing 7

The strength of this approach is that it's endorsed by both the 2019 AGA guidelines 1 and 2018 British Society of Gastroenterology guidelines 2, representing international consensus. The key is being selective rather than exhaustive - targeted testing based on predominant symptoms prevents unnecessary procedures while catching important treatable conditions like celiac disease, IBD, bile acid diarrhea, and microscopic colitis.