What is the best initial test for a patient presenting with bloody diarrhea in the Emergency Room (ER)?

Best Initial Test for Bloody Diarrhea in the Emergency Room

For patients presenting with bloody diarrhea in the emergency room, stool culture with specific testing for Shiga toxin-producing E. coli (STEC) is the best initial test. 1

Diagnostic Approach

Primary Testing

• Stool culture for bacterial pathogens including:

  • Salmonella
  • Shigella
  • Campylobacter
  • E. coli O157:H7
  • Other STEC

• Shiga toxin testing should be performed concurrently, as this is critical for detecting both O157 and non-O157 STEC strains 1

Supporting Evidence

• Bloody diarrhea significantly increases the likelihood of bacterial pathogens, particularly STEC
• 63% of specimens that yielded STEC O157 had gross blood and 91% were from patients with a history of bloody diarrhea 1
• Selective testing improves diagnostic yield - the CDC recommends E. coli O157 testing for all persons with acute bloody diarrhea 1

Additional Testing Based on Clinical Presentation

1. C. difficile testing if:

   • Recent antibiotic use
   • Healthcare exposure
   • Age >65 years 2

2. Specific bacterial testing based on epidemiology:

   • Vibrio testing if seafood consumption
   • Yersinia testing in certain at-risk populations 1

3. Laboratory tests to assess severity:

   • Complete blood count
   • Electrolytes
   • Renal function
   • C-reactive protein 1

Clinical Considerations

High-Risk Features Requiring Close Monitoring

• Infants <3 months of age
• Immunocompromised patients
• Severe abdominal pain
• Signs of dehydration
• Fever >38.5°C
• Significant bloody diarrhea 1, 2

Monitoring for Complications

• For patients with confirmed STEC, especially O157:H7:
  • Monitor hemoglobin, platelet counts, electrolytes, BUN, and creatinine frequently
  • Examine peripheral blood smear for RBC fragments if HUS is suspected 1

Important Caveats

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

2. Avoid empiric antimicrobial therapy for bloody diarrhea while awaiting test results, as antibiotics may increase the risk of HUS in STEC infections 1

3. Consider non-infectious causes if symptoms persist >14 days, particularly inflammatory bowel disease 1

4. Fresh diarrheal stool is the optimal specimen, providing significantly higher detection rates (49% vs 9%) compared to formed stool 2

5. Rapid molecular testing (PCR) should be considered when available as it can significantly reduce time to diagnosis (6.17 hours vs 57.28 hours for culture) 3

Bloody diarrhea should be treated as a medical emergency requiring prompt diagnostic evaluation to guide appropriate management and prevent complications 4.