What lab tests should be ordered for a patient presenting with huge infectious non-bloody diarrhea?

Laboratory Tests for Infectious Non-Bloody Diarrhea

For a patient with huge infectious non-bloody diarrhea, the optimal diagnostic specimen is a diarrheal stool sample that takes the shape of the container, which should be submitted for routine stool culture and molecular testing. 1

Primary Diagnostic Tests

First-Line Testing

• Stool Culture: For detection of bacterial pathogens including:

  • Salmonella species
  • Shigella species
  • Campylobacter species
  • E. coli O157:H7 or other Shiga toxin-producing E. coli 1

• Molecular Testing: Nucleic acid amplification tests (NAATs) for detection of:

  • Bacterial pathogens (more sensitive than culture)
  • Viral pathogens (norovirus, rotavirus, adenovirus)
  • Parasitic organisms 1

Additional Testing Based on Clinical Scenario

• Clostridium difficile testing: Consider in patients with:

  • History of antibiotic use
  • Healthcare-associated diarrhea
  • Persistent diarrhea without identified etiology 1

• Parasite Testing:

  • Ova and parasite examination including permanent stained smears
  • Specific immunoassays for Giardia lamblia, Cryptosporidium, and Entamoeba histolytica 1

Specimen Collection Guidelines

• Optimal specimen: Fresh diarrheal stool that takes the shape of the container 1
• Alternative: If timely diarrheal stool cannot be collected, a rectal swab may be used for bacterial detection (though less optimal) 1
• Timing: Collect specimen before antimicrobial administration when possible 2
• Volume: Submit sufficient quantity for complete testing (follow laboratory guidelines)

Testing Algorithm

1. Initial evaluation:

   • Submit fresh stool for routine enteric pathogen culture AND molecular testing (NAAT)
   • Consider multiplex PCR panels that detect multiple bacterial, viral, and parasitic pathogens simultaneously

2. If high-risk exposures or specific clinical features present:

   • Travel history → Add specific testing for enterotoxigenic E. coli, Vibrio, etc.
   • Immunocompromised status → Add testing for opportunistic pathogens
   • Persistent symptoms → Consider parasitic causes and C. difficile

3. For negative initial testing with ongoing symptoms:

   • Consider additional specialized testing for less common pathogens
   • Reevaluate for non-infectious causes 1

Important Considerations

• Do not use fecal leukocyte examination or stool lactoferrin detection to establish the cause of infectious diarrhea (strong recommendation) 1

• Do not use serologic tests to establish an etiology of infectious diarrhea (strong recommendation) 1

• Molecular techniques are generally more sensitive than culture-based methods and less dependent on specimen quality 1

• For viral and protozoal agents, and C. difficile toxin detection, fresh stool is strongly preferred over rectal swabs 1

Common Pitfalls to Avoid

• Inadequate specimen collection: Ensure proper collection of diarrheal (not formed) stool
• Delayed transport: Specimens should be transported promptly to the laboratory
• Premature cessation of testing: If initial tests are negative but symptoms persist, consider expanded testing
• Overreliance on single test modality: Combined approaches (culture plus molecular) yield higher detection rates
• Failure to consider C. difficile: Particularly in patients with recent antibiotic exposure or healthcare contact

By following this systematic approach to laboratory testing, clinicians can optimize the detection of infectious causes of non-bloody diarrhea, leading to appropriate treatment and improved patient outcomes.