What stool studies should be ordered for an adult patient presenting with acute diarrhea?

Stool Studies for Acute Diarrhea in Adults

For acute diarrhea in adults, order stool culture for the four primary bacterial pathogens (Salmonella, Shigella, Campylobacter, and E. coli O157:H7/STEC), C. difficile testing if there is recent antibiotic use, and consider multiplex molecular diagnostics as an alternative to traditional culture. 1

Core Testing Strategy

Bacterial Pathogens - Selective Approach

The diagnostic yield of routine stool cultures in acute diarrhea is low (1.5-5.8%), making selective testing critical. 2 Order stool culture or multiplex molecular diagnostics when patients have:

• Bloody diarrhea - 63% of STEC O157 specimens had gross blood, and 91% had a history of bloody diarrhea 2
• Fever >37.8°C - increases likelihood of positive culture 2.3-fold 3
• ≥5 episodes of diarrhea per day - increases likelihood 3.5-fold 3
• Elevated CRP >50 mg/L - increases likelihood 2.3-fold 3
• Severe volume depletion or signs of sepsis 4

When all three factors (fever, frequent diarrhea, elevated CRP) are present, the odds ratio for positive culture is 6.55. 3 Conversely, vomiting as a predominant symptom reduces the likelihood of bacterial infection. 3

Specific Pathogen Considerations

For bloody diarrhea specifically: Test for E. coli O157:H7/STEC using sorbitol-MacConkey (SMAC) agar or Shiga toxin EIA after broth enrichment. 2 This is critical because STEC can lead to hemolytic uremic syndrome (HUS). 2

For shellfish ingestion within 3 days: Culture on thiosulfate-citrate-bile salts (TCBS) medium for Vibrio species. 2

Single specimen is sufficient - multiple stool specimens do not increase diagnostic yield. 1 The optimal specimen is a diarrheal stool that takes the shape of the container. 1

C. difficile Testing

Test for C. difficile in any patient >2 years with diarrhea following recent antimicrobial use. 1 Use a two-step algorithm: 2, 1

1. First step: Glutamate dehydrogenase (GDH) enzyme immunoassay or nucleic acid amplification testing (NAAT/PCR) to confirm organism presence
2. Second step: Toxin EIA to demonstrate active toxin production

Never use toxin EIA alone - it has insufficient sensitivity. 1 This two-step approach provides high negative and positive predictive values when tests agree. 2 When results are discordant, clinical judgment determines treatment need. 2

The "3-Day Rule" for Hospitalized Patients

Do not order routine bacterial stool cultures for diarrhea developing >72 hours after hospital admission unless specific risk factors are present. 2, 5 The yield drops from 3.3% to 0.5% after 72 hours. 5

Exceptions to the 3-day rule - order cultures if the patient has: 5

• Age ≥65 years with comorbid disease
• Neutropenia
• HIV infection
• Community-acquired diarrhea that manifested after admission
• Nondiarrheal manifestations of enteric infection (e.g., fever, bacteremia)

Implementing this modified rule reduces stool cultures by 52% while missing no clinically significant cases. 5

Parasitic Testing

Test for Giardia specifically using stool antigen testing or PCR (sensitivity and specificity >95%). 2, 1 This is particularly important as Giardia is common throughout the United States and readily treatable. 2

Do not routinely order ova and parasite examination in patients without travel history to or recent immigration from high-risk areas - the yield is extremely low. 2 For travelers with diarrhea lasting ≥14 days, examination of three fresh stools for ova, cysts, and parasites has 60-90% sensitivity. 1

Additional Laboratory Work

Obtain complete blood count, comprehensive metabolic panel, and CRP in patients with severe symptoms or suspected systemic infection. 1 These help assess volume status, electrolyte abnormalities, and severity of inflammation.

Blood cultures should be obtained when enteric fever (typhoid) or bacteremia is suspected, particularly with high fever and systemic toxicity. 1

Common Pitfalls to Avoid

• Don't over-test mild cases - the majority of acute diarrhea is viral and self-limited, requiring only supportive care. 4 Testing in patients without severe symptoms is unrevealing and wasteful. 4
• Don't order multiple stool specimens - a single diarrheal sample is sufficient. 1
• Don't use the presence of fecal leukocytes and blood alone to guide culture decisions - while their combined presence has 81% sensitivity and 74% specificity for bacterial pathogens, clinical factors (fever, frequency, CRP) provide better prediction. 6, 3
• Don't forget C. difficile in any patient with recent antibiotic exposure, regardless of timing. 1