What are the diagnostic tests and treatment options for bowel infections?

Diagnostic Testing for Bowel Infections

For suspected bowel infections, obtain stool testing for bacterial pathogens (Salmonella, Shigella, Campylobacter, Yersinia, STEC) and C. difficile toxin when patients present with fever, bloody/mucoid stools, severe abdominal cramping, or signs of sepsis. 1

Initial Clinical Assessment

History and Physical Examination

Document the following specific details:

• Stool frequency, consistency, presence of blood, mucus, or pus 2
• Fever, abdominal pain severity and location, tenesmus, urgency 2
• Recent travel history, shellfish consumption, antibiotic use within 8-12 weeks 2, 1
• Immunosuppression status, recent hospitalization (>3 days triggers different testing) 1
• Vital signs including temperature, pulse, blood pressure, weight 2, 3
• Abdominal examination for distension, tenderness, reduced bowel sounds 2
• Digital rectal examination to assess for blood 2, 3

Laboratory Testing Algorithm

Initial Blood Work

Obtain the following for all patients with suspected infectious colitis:

• Complete blood count (assess for leukocytosis, anemia, left shift) 2, 3
• Electrolytes, renal function, liver function tests 2, 3
• C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) 2
• Iron studies and vitamin D level 2

Stool Testing Strategy

The testing approach depends on clinical presentation and timing:

For acute diarrhea with inflammatory features (fever, bloody stools, severe cramping):

• Test for Salmonella, Shigella, Campylobacter, Yersinia, and Shiga toxin-producing E. coli (STEC) 2, 1
• C. difficile toxin assay 2, 1
• Multiplex PCR panels are acceptable alternatives to traditional culture, offering faster results 3
• Important caveat: Molecular tests detect DNA, not necessarily viable organisms, so interpret positive results in clinical context 2, 1

For patients hospitalized >3 days:

• Test only for C. difficile, not routine bacterial pathogens (the "3-day rule") 1

For recent antibiotic exposure (within 8-12 weeks):

• Prioritize C. difficile testing 2, 1

For persistent diarrhea (≥14 days):

• Test for parasitic infections: Cryptosporidium, Giardia, Cyclospora, Entamoeba histolytica 2, 1
• Ova and parasite examination based on travel history or endemic area residence 2, 3
• Consider Strongyloides serology if travel history suggests exposure 2

For immunocompromised patients:

• Broader testing including Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex 2
• Test for cytomegalovirus (CMV) in steroid-resistant disease, particularly in ulcerative colitis patients 2

Fecal Inflammatory Markers

• Fecal calprotectin correlates well with intestinal inflammation and helps distinguish inflammatory from non-inflammatory diarrhea 2
• Useful for monitoring disease activity and response to treatment 2

Endoscopic Evaluation

Sigmoidoscopy or Colonoscopy Indications:

• Positive stool inflammatory markers with uncertain diagnosis 3
• Severe symptoms or treatment-refractory disease 2, 3
• Need for histopathological confirmation 2

Critical safety consideration: In moderate-to-severe disease, flexible sigmoidoscopy is safer than colonoscopy due to lower perforation risk 2, 3

Obtain mucosal biopsies even if macroscopic appearance is normal, as microscopic changes may be present 2

Imaging Studies

Abdominal Radiography:

• Essential in suspected severe inflammatory bowel disease to exclude colonic dilatation, toxic megacolon, or perforation 2, 3

CT Abdomen/Pelvis:

• Indicated for severe colitis with fever, significant pain, bleeding, or concern for complications 3

Common Pitfalls to Avoid

• Do not test all diarrheal stools indiscriminately - this leads to unnecessary costs and low diagnostic yield 1
• Do not miss C. difficile in patients with recent antibiotic exposure - this is a critical diagnostic consideration 1
• Do not perform routine bacterial cultures on patients hospitalized >3 days - test only for C. difficile 1
• Do not rely solely on positive molecular tests without clinical correlation - these detect DNA, not necessarily active infection 2, 1
• Do not forget to culture positive molecular specimens when needed for antimicrobial susceptibility testing and public health surveillance 2, 1
• Do not overlook parasitic infections in patients with travel history or endemic area residence 2

Specimen Collection

The optimal specimen is a diarrheal stool sample (takes the shape of container) 2. If unavailable, rectal swabs may be used for bacterial detection, though molecular techniques are less dependent on specimen quality 2. Test unformed stool samples only 1.