What labs are recommended for patients presenting with diarrhea?

Recommended Laboratory Tests for Patients with Diarrhea

For patients presenting with diarrhea, recommended laboratory tests include stool cultures for bacterial pathogens, stool examination for ova and parasites (particularly Giardia), blood tests including complete blood count, C-reactive protein, and serological testing for celiac disease. 1

Initial Laboratory Evaluation

Stool Studies

• Stool culture for bacterial pathogens (Salmonella, Shigella, Campylobacter) 1
• Specific testing for Giardia (strongly recommended regardless of travel history) 1
• Clostridioides difficile testing for patients with recent antibiotic use or healthcare exposure 1
  • Two-stage approach recommended: glutamate dehydrogenase enzyme immunoassay (EIA) or PCR followed by toxin EIA 1
• Fecal calprotectin to screen for inflammatory bowel disease 1
• Stool ova and parasites only for patients with travel history to or recent immigration from high-risk areas 1

Blood Tests

• Complete blood count - to assess for anemia, leukocytosis/leukopenia 1
• C-reactive protein (though limited utility for IBD screening) 1
• Erythrocyte sedimentation rate (limited utility) 1
• Celiac disease screening:
  • IgA tissue transglutaminase (tTG) 1
  • Second test for IgA deficiency (IgG tTG or IgG/IgA deamidated gliadin peptides) 1
• Basic metabolic panel - to assess electrolyte disturbances and renal function 1
• Liver function tests 1
• Iron studies, vitamin B12, folate - to assess malabsorption 1
• Thyroid function tests - to rule out hyperthyroidism as a cause 1

Testing Based on Clinical Presentation

For Acute Infectious Diarrhea

• Focus on stool cultures and C. difficile testing 1
• Molecular diagnostic panels may be considered for more rapid diagnosis 1
• Serologic tests are not recommended to establish etiology of infectious diarrhea 1

For Chronic Diarrhea (>4 weeks)

• More comprehensive testing including:
  • Bile acid malabsorption testing if available (or consider empiric trial of bile acid binders) 1
  • Fecal elastase if fat malabsorption is suspected 1
  • Lactose hydrogen breath testing if lactose maldigestion is suspected 1

For Bloody Diarrhea

• Immediate stool culture with specific request for STEC/E. coli O157:H7 1
• Hemoglobin and platelet monitoring for patients with confirmed STEC infection to detect early HUS 1
• Peripheral blood smear when HUS is suspected 1

Special Populations

Immunocompromised Patients

• More extensive parasitology testing including Cryptosporidium, microsporidia 1
• HIV testing should be considered 1

Cancer Patients

• Additional tests may include:
  • Procalcitonin to assess for infection 1
  • Blood cultures if fever present, especially with neutropenia 1
  • Blood gases and lactate if severe illness 1

Follow-up Testing

• Follow-up testing is generally not recommended after resolution of diarrhea for clinical management 1
• Exception: Public health requirements may necessitate follow-up cultures for certain pathogens (Salmonella Typhi/Paratyphi, STEC, Shigella) 1
• For persistent symptoms (>14 days), consider non-infectious causes including IBD and IBS 1
• Endoscopic evaluation should be considered for persistent unexplained diarrhea, especially in patients with AIDS or other underlying medical conditions 1

Common Pitfalls to Avoid

1. Over-reliance on fecal leukocyte examination - not recommended for establishing cause of infectious diarrhea 1
2. Unnecessary testing for ova and parasites in patients without relevant travel history 1
3. Failure to test for C. difficile in patients with recent antibiotic exposure 1
4. Missing celiac disease - ensure appropriate serological testing 1
5. Inadequate monitoring for complications in patients with STEC infection 1
6. Overlooking factitious diarrhea - consider laxative screening in appropriate clinical settings 1

By following this systematic approach to laboratory testing for diarrhea, clinicians can efficiently identify the underlying cause and implement appropriate treatment to reduce morbidity and mortality.