What is the best initial testing approach for acute diarrhea?

Diagnostic Testing for Acute Diarrhea

For most cases of uncomplicated acute diarrhea, no diagnostic testing is recommended as the condition is typically self-limited and resolves without specific intervention. 1

When to Test

Testing should be performed in specific clinical scenarios:

• Severe illness: Fever, bloody/mucoid stools, severe abdominal cramping, or signs of sepsis
• High-risk patients: Immunocompromised, elderly, or those with underlying medical conditions
• Prolonged symptoms: Diarrhea lasting >14 days
• Specific exposures: Healthcare-associated diarrhea, travel history, or outbreak settings

Optimal Testing Approach

First-line Testing

• Optimal specimen: A diarrheal stool sample that takes the shape of the container 1
• For bacterial pathogens: Test for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC in symptomatic hosts 1
• Testing modality: Culture-independent diagnostic tests (CIDTs) such as multiplex PCR panels offer rapid results with higher sensitivity than traditional culture 2, 3

Special Circumstances Testing

1. For large-volume "rice water" stools or seafood exposure:

   • Add testing for Vibrio species 1, 2

2. For persistent abdominal pain, especially in school-aged children:

   • Test for Yersinia enterocolitica 1

3. For healthcare-associated diarrhea (>3 days after hospitalization):

   • Focus on C. difficile testing rather than routine stool culture 2
   • Test patients >2 years of age with history of antimicrobial use 1

4. For immunocompromised patients, especially those with AIDS:

   • Broader testing for Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and cytomegalovirus 1

5. For travelers with diarrhea lasting >14 days:

   • Evaluate for intestinal parasitic infections 1

6. For suspected enteric fever or bacteremia:

   • Perform blood cultures in addition to stool testing 1

Tests to Avoid

• Fecal leukocyte examination and stool lactoferrin detection should not be used to establish the cause of acute infectious diarrhea (strong recommendation) 1
• Serologic tests are not recommended to establish an etiology of infectious diarrhea or enteric fever 1
• Routine stool cultures in hospitalized patients after 3 days of admission have very low yield 2

Interpretation Considerations

• Molecular tests detect DNA, not necessarily viable organisms, which can lead to overtreatment if results are misinterpreted 1, 2
• A positive test result may not always indicate the causative agent, as asymptomatic carriage of some pathogens is possible 3
• Multiple pathogens may be detected in up to 21% of cases, complicating interpretation 3

Follow-up Testing

• Follow-up testing is generally not recommended for case management after resolution of symptoms 1
• For patients who fail to improve:
  • Consider noninfectious causes including lactose intolerance, IBD, and IBS 1
  • Clinical and laboratory reevaluation may be indicated 1
  • For symptoms lasting ≥14 days without identified cause, consider IBD and IBS 1

Pitfalls to Avoid

• Indiscriminate testing is costly and inefficient; selective testing based on clinical presentation improves diagnostic yield 4
• Overreliance on traditional stool culture alone may miss important pathogens; consider multiplex molecular testing for comprehensive coverage 3, 5
• Failing to consider C. difficile in community-acquired diarrhea (up to 60% of C. difficile cases may be community-acquired) 3
• Misinterpreting molecular test results without considering clinical context can lead to unnecessary treatment 2

By following this targeted approach to diagnostic testing for acute diarrhea, clinicians can improve diagnostic yield while reducing unnecessary testing and inappropriate antimicrobial use.