What are the recommended tests for a patient presenting with high-risk acute diarrhea, particularly if they have recently been on antibiotics or show signs of severe dehydration or systemic illness?

High-Risk Acute Diarrhea Testing

For patients with high-risk acute diarrhea, order stool testing for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC when they present with fever ≥38.5°C, bloody or mucoid stools, severe abdominal cramping, signs of sepsis, recent antibiotic use, or belong to high-risk populations. 1, 2

Clinical Indicators for Stool Testing

Inflammatory Markers Requiring Testing

• Fever: Documented temperature ≥38.5°C warrants stool cultures 2, 3
• Bloody or mucoid stools: Always test for bacterial pathogens including STEC 1, 2, 3
• Severe abdominal cramping or tenderness: Indicates potential invasive pathogen requiring culture 2, 4
• Signs of sepsis: Tachycardia, hypotension, or altered mental status mandate immediate testing and blood cultures 2, 3

Recent Antibiotic Exposure

• Test for C. difficile in any patient with diarrhea who received antibiotics within the previous 8-12 weeks 1, 2, 5
• Submit a single diarrheal stool specimen for C. difficile toxin assay initially 1
• If diarrhea persists and initial assay is negative, submit 1-2 additional specimens 1
• C. difficile yields are 15-20% in patients with healthcare-associated diarrhea or recent antibiotic use 1

High-Risk Populations Requiring Comprehensive Testing

Age-Based Risk

• Infants <3 months: Always perform stool cultures and blood cultures regardless of other factors 2
• Age >65 years: Consider cultures even if diarrhea develops >3 days after hospitalization 1

Immunocompromised Patients

• Require broad evaluation including bacterial culture, viral studies, and parasitic examination 2, 5
• This includes HIV patients, those undergoing chemotherapy, transplant recipients, and patients with neutropenia 1, 2
• Consider duodenal aspirate for Giardia, Strongyloides, Cystoisospora, or microsporidia in select cases 1

Additional High-Risk Conditions

• Hemolytic anemia (concern for STEC/HUS) 2
• Suspected enteric fever (travel to endemic areas) 2
• Healthcare-associated diarrhea developing >3 days after hospitalization 1

Epidemiologic Triggers for Specific Testing

Travel and Exposure History

• Vibrio species: Large volume rice-water stools, brackish water exposure, raw shellfish consumption, or travel to cholera-endemic regions within 3 days 2
• Yersinia enterocolitica: School-aged children with right lower quadrant pain mimicking appendicitis or infants exposed to raw/undercooked pork 2

Outbreak Settings

• Multiple people with diarrhea sharing a common meal or sudden rise in cases requires coordination with public health authorities and stool testing 2

Optimal Specimen Collection

Specimen Type

• A diarrheal stool sample (one that takes the shape of the container) is the optimal specimen 1, 5
• If timely diarrheal stool cannot be collected, a rectal swab may be used for bacterial detection, though molecular techniques are less dependent on specimen quality 1, 5
• Fresh stool is preferred for viral, protozoal agents, and C. difficile toxin 1

Number of Specimens

• A single diarrheal stool specimen is recommended for most bacterial, viral, and protozoal testing 1, 5
• Multiple specimens are unnecessary and not cost-effective 1

Blood Culture Indications

Obtain blood cultures in the following scenarios:

• Infants <3 months with diarrhea 2
• Any age with signs of septicemia or systemic manifestations 2, 5
• Suspected enteric fever 2
• Immunocompromised patients 2

STEC-Specific Approach

• Use methods that detect Shiga toxin and distinguish E. coli O157:H7 from other STEC serotypes 5
• Employ Sorbitol-MacConkey agar or chromogenic agar to screen for O157:H7 2
• Use genomic assays for non-O157 STEC detection 2

Critical Pitfalls to Avoid

• Never administer antibiotics before testing in suspected STEC cases, as this increases hemolytic uremic syndrome risk 2
• Do not use fecal leukocyte examination or stool lactoferrin to establish the cause of acute infectious diarrhea—these tests should not guide testing decisions 1, 5
• Do not test asymptomatic patients or perform follow-up testing after symptom resolution for case management 1, 6
• Avoid multiple stool examinations for ova and parasites in hospitalized patients with nosocomial diarrhea (low yield) 1
• Do not culture specimens from patients hospitalized >3 days for standard bacterial pathogens unless they were admitted for diarrheal illness, are part of an outbreak, or have specific risk factors (age >65, comorbidities, neutropenia, HIV) 1