What is the differential diagnosis and initial management of acute diarrhea?

Differential Diagnosis and Initial Management of Acute Diarrhea

The differential diagnosis of acute diarrhea is primarily divided into infectious causes (viral, bacterial, parasitic) and non-infectious causes, with the initial approach focused on assessing dehydration status, obtaining a targeted clinical history, and determining whether stool testing is warranted based on specific clinical features and risk factors. 1

Clinical Classification Framework

Acute diarrhea is defined as lasting 0-13 days and should be categorized as either inflammatory or noninflammatory based on clinical presentation 1, 2:

Inflammatory Diarrhea (Bloody/Mucoid Stools)

Bacterial causes:

• Salmonella, Shigella, Campylobacter - most common invasive bacterial pathogens requiring stool testing 1, 3
• STEC (E. coli O157:H7 and non-O157) - critical to identify due to hemolytic uremic syndrome risk 1
• Yersinia enterocolitica - suspect in school-aged children with right lower quadrant pain mimicking appendicitis or infants exposed to raw/undercooked pork 1, 3
• Clostridium difficile - consider in patients with antibiotic exposure within 8-12 weeks 4, 3

Parasitic causes:

• Entamoeba histolytica - causes persistent diarrhea with visible blood and mucus 4

Non-infectious inflammatory causes:

• Inflammatory bowel disease (ulcerative colitis, Crohn's disease) - presents with mucoid diarrhea, rectal bleeding, urgency, and abdominal pain 4

Noninflammatory Diarrhea (Watery Stools)

Viral causes (most common):

• Rotavirus, norovirus, adenovirus - typically self-limited 5, 2

Bacterial causes:

• Vibrio species - suspect with large volume rice-water stools, exposure to brackish water, raw shellfish consumption, or travel to cholera-endemic regions within 3 days 1, 3
• Enterotoxigenic E. coli - common in travelers' diarrhea 6

Parasitic causes (persistent >14 days):

• Giardia - associated with weight loss in 75% of cases 4, 7
• Cryptosporidium - weight loss in 75% of cases 4, 7
• Cyclospora - weight loss in 91% of cases 4, 7

Initial Assessment Algorithm

Step 1: Evaluate for Dehydration (PRIORITY)

All patients with acute diarrhea must be assessed for dehydration, which increases risk of life-threatening illness and death, especially in young children and older adults 1. Signs include:

• Tachycardia, hypotension, altered mental status 3
• Decreased skin turgor, dry mucous membranes 5
• Reduced urine output 5

Step 2: Obtain Targeted Clinical History

A detailed clinical and exposure history is mandatory 1:

• Stool characteristics: frequency, volume, presence of blood/mucus 1, 4
• Fever: documented temperature ≥38.5°C indicates need for testing 3
• Abdominal symptoms: severe cramping, tenderness, right lower quadrant pain 1, 3
• Epidemiologic exposures:
  • Recent antibiotic use (8-12 weeks) - suspect C. difficile 4, 3
  • Raw/undercooked pork - suspect Yersinia 1, 3
  • Raw shellfish or brackish water exposure - suspect Vibrio 1, 3
  • Travel to endemic areas - suspect enteric fever, cholera 1, 3
  • Daycare, long-term care, food service, healthcare settings - outbreak potential 1
• Immunocompromise status: HIV, chemotherapy, transplant 3
• Age: infants <3 months require blood cultures 1, 3

Step 3: Determine Need for Stool Testing

Stool testing for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC is indicated when ANY of the following are present 1, 3:

• Fever (≥38.5°C) 3
• Bloody or mucoid stools 1, 3
• Severe abdominal cramping or tenderness 1, 3
• Signs of sepsis 1, 3
• Infants <3 months of age 1, 3
• Immunocompromised patients 1, 3
• Suspected enteric fever or outbreak setting 1, 3

For patients WITHOUT these features, stool testing is NOT routinely indicated 2. Most cases are viral and self-limited 2.

Diagnostic Testing Approach

Stool Testing Methods

• Single diarrheal stool specimen is optimal - rectal swab acceptable if stool unavailable 4, 7
• Molecular multiplex PCR tests are preferred over traditional cultures - higher sensitivity for multiple pathogens simultaneously 4, 3
• STEC-specific testing: Use Sorbitol-MacConkey agar or chromogenic agar to screen for O157:H7; detect Shiga toxin or genomic assays for non-O157 STEC 1, 3
• Distinguish Shiga toxin 1 from toxin 2 if available (toxin 2 is more potent) 1

Blood Cultures Indicated For:

• Infants <3 months of age 1, 3
• Signs of septicemia or systemic manifestations 1, 3
• Suspected enteric fever 1, 3
• Immunocompromised patients 1, 3
• High-risk conditions (hemolytic anemia) 1, 3

Special Testing Considerations

• Persistent diarrhea (≥14 days) with weight loss: Test for Giardia, Cryptosporidium, Cyclospora 4, 7, 8
• Immunocompromised patients: Broader evaluation including bacterial culture, viral studies, and parasitic examination 3

Initial Management

Rehydration (FIRST-LINE THERAPY FOR ALL PATIENTS)

Oral rehydration solution is the cornerstone of treatment 4, 9:

• WHO-recommended formulation: Na 90 mM, K 20 mM, glucose 111 mM 4
• Oral rehydration preferred for all patients able to take oral fluids 4
• Intravenous rehydration warranted for severe dehydration or sepsis 2

Symptomatic Therapy

Antidiarrheal agents (opiate derivatives, bismuth subsalicylate) can reduce bowel movements and fluid loss in watery diarrhea 9, 2:

• Useful for symptomatic relief and decreasing inappropriate antibiotic use 2
• AVOID in bloody/mucoid diarrhea or suspected STEC 4

Antibiotic Therapy

Empirical antibiotics should NOT be routinely used in immunocompetent patients while awaiting test results 4, 3:

Exceptions warranting empirical treatment:

• Infants <3 months with suspected bacterial etiology 4
• Documented fever with bloody diarrhea suggesting bacillary dysentery (Shigella) 4
• Signs of sepsis 3, 2
• Suspected enteric fever 1

Targeted antibiotic therapy should be based on stool testing results 4, 2

Critical Pitfalls to Avoid

1. NEVER give antibiotics empirically in suspected STEC - increases hemolytic uremic syndrome risk 4, 3
2. Do not forget C. difficile testing in patients with recent antibiotic exposure (8-12 weeks) 4, 7
3. Do not overlook parasitic causes in persistent diarrhea >14 days, especially with weight loss 4, 7
4. Do not perform colonoscopy in moderate-severe inflammatory disease - perforation risk 4
5. Do not routinely culture stool in uncomplicated watery diarrhea without risk factors - low yield and high cost 6, 2
6. Evaluate for postinfectious and extraintestinal manifestations (reactive arthritis, hemolytic uremic syndrome) 1

Public Health Considerations

Patients working in childcare, long-term care, patient care, food service, or recreational water venues must follow jurisdictional outbreak reporting and infection control recommendations 1