What stool test should be ordered for a patient, potentially an adult, presenting with a three-week history of diarrhea, considering potential bacterial, parasitic, or Clostridioides difficile (C. diff) infections?

Stool Testing for Three Weeks of Diarrhea

For a patient with three weeks of diarrhea, order stool testing for Giardia (antigen or PCR), celiac serology (IgA tissue transglutaminase), and consider fecal calprotectin to screen for inflammatory bowel disease. 1

Core Testing Strategy

Essential Tests

• Giardia testing is mandatory for all patients with chronic diarrhea (>14 days), as this is the most common parasitic cause and can become chronic after the typical 2-6 week course 1, 2

  • Use Giardia antigen immunoassay or nucleic acid amplification testing (NAAT/PCR), which are more sensitive than traditional microscopy 1
  • Traditional ova and parasite (O&P) examination requires 3 stool samples collected on different days and is labor-intensive with low yield 1

• Celiac disease screening with IgA tissue transglutaminase (tTG) plus total IgA (or alternative test if IgA deficient) is strongly recommended 1

  • This has >90% sensitivity and specificity for celiac disease, a critical treatable cause of chronic diarrhea 1

• Fecal calprotectin or fecal lactoferrin to screen for inflammatory bowel disease (IBD) 1

  • More useful than serum inflammatory markers (ESR/CRP) for detecting intestinal inflammation 1

Conditional Testing Based on Risk Factors

C. difficile testing should be ordered if: 1

• Recent antibiotic use within the past 8-12 weeks 1
• Healthcare-associated diarrhea or recent hospitalization 1
• Use a single diarrheal stool specimen for toxin or toxigenic strain detection (NAAT preferred) 1
• Multiple specimens do not increase yield 1

Expanded bacterial stool culture (beyond routine pathogens) only if: 1

• Persistent symptoms despite negative initial workup
• Travel to endemic areas
• Immunocompromised status
• The yield is extremely low (1.4%) in immunocompetent patients without travel history 1

Avoid routine ova and parasite (O&P) testing unless specific risk factors present: 1

• Recent travel to or immigration from endemic areas
• Men who have sex with men
• Exposure to contaminated water sources
• Day-care exposure
• HIV-positive status 2, 3

What NOT to Order

• Do not order routine bacterial stool cultures for patients hospitalized >3 days or with chronic diarrhea without fever/bloody stools 1, 4

  • The positivity rate drops from 12.6% to 1.4% after 3 days of hospitalization 4

• Do not order broad O&P panels without specific risk factors, as the yield is only 2.15% in unselected populations and costs approximately $1,836 per positive result 1, 3

• Do not order multiple stool specimens for C. difficile - a single specimen is sufficient 1

Additional Considerations for Bile Acid Diarrhea

Consider bile acid diarrhea testing in patients with watery diarrhea and negative initial workup: 1

• 48-hour stool collection for total bile acids (available in US)
• Serum FGF19 (fibroblast growth factor 19) measurement
• SeHCAT testing (not available in US but gold standard in Europe) 1
• Alternatively, empiric trial of bile acid sequestrants (cholestyramine) may be reasonable given limited test availability 1

Common Pitfalls to Avoid

• Don't reflexively order comprehensive stool panels - they detect DNA, not necessarily viable organisms, and can lead to overdiagnosis 1
• Don't test asymptomatic patients or repeat testing after treatment unless for public health surveillance 5
• Don't order serology for enteric fever in patients with chronic diarrhea without fever or travel history 1
• Remember that chronic diarrhea (>14 days) has different etiologies than acute diarrhea - infectious causes are rare in immunocompetent patients without risk factors 1