Which laboratory tests should be ordered for suspected infectious colitis?

Laboratory Testing for Suspected Infectious Colitis

For patients with suspected infectious colitis presenting with fever, bloody or mucoid stools, severe abdominal cramping, or signs of sepsis, order stool testing for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC (Shiga toxin-producing E. coli), along with blood cultures in specific high-risk populations. 1

Core Stool Testing Panel

The foundation of laboratory evaluation requires comprehensive stool testing that includes:

• Bacterial pathogens: Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC 1
• STEC-specific testing: E. coli O157:H7 should be assessed by culture using sorbitol-MacConkey agar or chromogenic agar, while non-O157 STEC requires Shiga toxin detection or genomic assays 1
• Optimal specimen: A diarrheal stool sample (one that takes the shape of the container) is preferred; rectal swabs may be used if timely stool collection is not possible 1

Important caveat: Bloody stools are NOT an expected manifestation of C. difficile infection, so if bloody diarrhea is present without recent antibiotic exposure, prioritize testing for other bacterial pathogens first 1

Blood Cultures - When to Order

Blood cultures are mandatory in these specific scenarios 1:

• Infants <3 months of age (regardless of other factors)
• Any age with signs of septicemia or systemic manifestations
• Suspected enteric fever (especially with travel history to endemic areas)
• Immunocompromised patients
• High-risk conditions such as hemolytic anemia
• Travelers from enteric fever-endemic areas with febrile illness of unknown etiology

Epidemiologic-Driven Additional Testing

Yersinia Testing

Order when 1:

• Persistent abdominal pain, especially in school-aged children with right lower quadrant pain mimicking appendicitis (mesenteric adenitis)
• Fever with epidemiologic risk (infants with exposure to raw or undercooked pork products)

Vibrio Testing

Order when 1:

• Large volume "rice water" stools
• Exposure to salty or brackish waters
• Consumption of raw or undercooked shellfish
• Travel to cholera-endemic regions within 3 days prior to diarrhea onset

Parasitic Testing

Order when 1:

• Diarrhea lasting ≥14 days (especially in travelers)
• Immunocompromised patients require broad parasitic evaluation including Cryptosporidium, Cyclospora, Cystoisospora, microsporidia 1

Special Population Considerations

Immunocompromised Patients

Require expanded testing including 1:

• Stool culture, viral studies, and parasitic examination
• AIDS patients with persistent diarrhea: Add testing for Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and cytomegalovirus

Recent Antibiotic Exposure

• Test for C. difficile in patients with diarrhea following antimicrobial use within the preceding 8-12 weeks 1
• Consider C. difficile testing in healthcare-associated diarrhea 1

Testing Methodology Preferences

Multiplex molecular diagnostics (NAAT) are now preferred over traditional stool cultures 1, 2, 3:

• Nucleic acid amplification tests detect DNA/genes of pathogens
• More sensitive and less dependent on specimen quality than culture
• Critical caveat: These assays detect DNA, not necessarily viable organisms, so clinical context is essential for interpretation 1

However, culture-dependent testing remains valuable for 1:

• Antibiotic susceptibility testing
• Subtyping and Whole Genome Sequencing
• Public health surveillance

Outbreak Scenarios

When multiple people share symptoms or a common meal exposure 1:

• Expand testing to include broader bacterial, viral, and parasitic agents regardless of fever or bloody stools
• Coordinate testing selection with public health authorities
• Consider viral pathogens (norovirus, rotavirus, enteric adenovirus) using NAAT 1

Common Pitfalls to Avoid

• Don't order C. difficile testing in children <2 years of age without clear indication, as colonization is common 1
• Don't use serologic tests to diagnose enteric fever—they are unreliable 1
• Don't forget to assess for dehydration in all patients with acute diarrhea, as this increases mortality risk, especially in young children and elderly 1
• Don't use antimotility agents in suspected STEC infection due to risk of hemolytic uremic syndrome 1