Diagnostic Approach to Diarrhea
Patient Selection for Stool Studies
- For patients with diarrhea, stool testing should be performed for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and Shiga toxin-producing E. coli (STEC) when accompanied by fever, bloody or mucoid stools, severe abdominal cramping or tenderness, or signs of sepsis, as recommended by the Infectious Diseases Society of America 1, 2
- Patients with diarrhea lasting >14 days should be evaluated for parasitic infections, such as Cryptosporidium, Giardia, Cyclospora, and Entamoeba histolytica, with a strength of evidence rated as moderate 4
- Patients who develop diarrhea after 3 days of hospitalization should be tested for C. difficile rather than routine bacterial pathogens, following the "3-day rule" with high strength of evidence 5, 6
- Patients who have recently taken antibiotics (within 8-12 weeks) should be tested for C. difficile, as recommended by the Infectious Diseases Society of America with moderate strength of evidence 4, 7
Recommended Stool Studies Based on Clinical Presentation
- For acute diarrhea with inflammatory features, test for Salmonella, Shigella, Campylobacter, Yersinia, and STEC, as recommended by the Infectious Diseases Society of America with high strength of evidence 1, 2
- For STEC detection, use methods that detect Shiga toxin (or genes that encode them) and distinguish E. coli O157:H7 from other STEC serotypes, with moderate strength of evidence 2, 8
- For persistent diarrhea (14-29 days), test for parasitic infections including Cryptosporidium, Giardia, Cyclospora, and Entamoeba histolytica, as recommended by the Infectious Diseases Society of America with moderate strength of evidence 4, 8
Diagnostic Methods
- Traditional stool culture remains important for antimicrobial susceptibility testing and public health surveillance, with high strength of evidence 4
- Specimens that test positive by molecular methods should be cultured if required for public health reporting or antimicrobial susceptibility testing, as recommended by the Infectious Diseases Society of America with moderate strength of evidence 4
- C. difficile toxin detection is the primary diagnostic approach, with high strength of evidence 7
- Testing should be performed on unformed stool samples, as recommended by the European Society of Clinical Microbiology and Infectious Diseases with moderate strength of evidence 7
Common Pitfalls to Avoid
- Testing all diarrheal stools without consideration of clinical presentation leads to unnecessary costs and low yield, as warned by the Infectious Diseases Society of America with high strength of evidence 5, 6
- Failing to test for C. difficile in patients with recent antibiotic exposure, as recommended by the Infectious Diseases Society of America with moderate strength of evidence 4, 7
- Misinterpreting molecular test results - these detect DNA, not necessarily viable organisms, with moderate strength of evidence 4