Stool Studies for Diarrhea: Diagnostic and Treatment Approach
When to Order Stool Studies
Order stool testing when diarrhea persists ≥8 days, visible blood is present, fever occurs with bloody diarrhea documented in a medical setting, severe abdominal pain accompanies diarrhea, the patient is immunocompromised with moderate-to-severe symptoms, or recent international travel occurred within 3 days of symptom onset. 1
Additional indications include:
- Suspected outbreak settings with multiple people sharing exposure 1
- Recent antibiotic use within 8-12 weeks (triggers C. difficile testing) 1
- Bloody diarrhea specifically (requires STEC testing) 1
Optimal Specimen Collection
- A single diarrheal stool specimen is the optimal sample for laboratory diagnosis 1, 2
- Multiple specimens are unnecessary and increase cost without improving diagnostic yield 1
Initial Laboratory Workup
Standard Stool Testing Panel
Order the following on a single stool specimen:
- Bacterial culture for Salmonella, Shigella, Campylobacter, and Yersinia 1, 2
- C. difficile testing if antibiotic use within 8-12 weeks 1, 2
- STEC (Shiga toxin-producing E. coli) testing if bloody diarrhea present 1, 2
- Giardia testing, as it commonly causes persistent diarrhea 2
Additional Laboratory Tests
- Complete blood count (CBC) and basic metabolic panel (BMP) to assess inflammation, dehydration, and electrolyte abnormalities 2
- Ova and parasite examination for patients with recent travel history 2
Empiric Antibiotic Treatment Decisions
When to Withhold Empiric Antibiotics
Do NOT give empiric antibiotics for bloody diarrhea in immunocompetent patients while awaiting results, except in specific high-risk scenarios 1
When Empiric Treatment IS Indicated
Treat empirically in these situations:
- Infants <3 months with suspected bacterial etiology 1
- Ill patients with documented fever, abdominal pain, bloody diarrhea, and bacillary dysentery 1
- Recent international travelers with temperature ≥38.5°C or signs of sepsis 1
- Immunocompromised patients with severe illness 1
Antibiotic Selection
- Fluoroquinolone (ciprofloxacin) or azithromycin based on local resistance patterns and travel history 1
- Azithromycin is increasingly preferred due to rising fluoroquinolone resistance, particularly among Campylobacter 3
- For acute watery diarrhea: azithromycin 500 mg single dose or ciprofloxacin 750 mg single dose 3
- For febrile diarrhea/dysentery: azithromycin 1,000 mg single dose 3
Supportive Care Measures
- Reduced osmolarity oral rehydration solution (ORS) as first-line therapy for mild-to-moderate dehydration 1
- Loperamide may be used as adjunctive therapy with antibiotics for moderate-to-severe diarrhea, or as monotherapy for moderate diarrhea 1
Follow-Up for Persistent Symptoms
At 7-8 Days
- Consider diagnostic stool testing if symptoms persist beyond expected duration or fail to respond to initial therapy 1
- Most viral diarrhea resolves within 3-7 days; persistence beyond this warrants bacterial and parasitic evaluation 4
At 14 Days
- Evaluate for parasitic infections (Giardia, Cryptosporidium) and consider non-infectious causes 1, 2
- Non-infectious causes include inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), and lactose intolerance 1, 2
- Consider bile acid diarrhea in patients with persistent watery diarrhea 2
Beyond 30 Days
- Evaluate for chronic non-infectious causes including microscopic colitis and postinfectious IBS 4
Common Pitfalls to Avoid
- Do not routinely order stool studies for mild, self-limited diarrhea - the majority of acute diarrhea cases are viral and self-limiting within 3-7 days 5
- Do not use multiple stool specimens - this increases cost without improving diagnostic yield 1
- Do not give empiric antibiotics to all patients with bloody diarrhea - this can worsen outcomes in certain infections like STEC and contributes to antibiotic resistance 1, 6
- Do not forget to test for C. difficile in patients with recent antibiotic exposure - this is a frequently missed diagnosis 1, 7