What laboratory tests are indicated for the evaluation of diarrhea?

From the Guidelines

For diarrhea evaluation, I recommend ordering stool tests for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC, as well as blood cultures in certain high-risk individuals, such as infants under 3 months, people with signs of septicemia, or those who are immunocompromised 1. When evaluating a patient with diarrhea, it is essential to consider their clinical presentation, including the presence of fever, bloody or mucoid stools, severe abdominal cramping, or signs of sepsis.

• Stool testing should be performed for patients with these symptoms, as well as for those with certain epidemiologic risk factors, such as exposure to raw or undercooked pork products or travel to enteric fever-endemic areas 1.
• Blood cultures should be obtained from high-risk individuals, including infants under 3 months, people with signs of septicemia, or those who are immunocompromised 1. Additional tests to consider based on clinical presentation include:
• Fecal occult blood test to detect bloody stools
• Stool ova and parasite examination to detect organisms such as Giardia and Entamoeba
• Clostridioides difficile toxin assay, particularly in patients with recent antibiotic use or healthcare exposure
• Stool PCR for enteric pathogens, such as Vibrio species in people with large volume rice water stools or exposure to salty or brackish waters 1. The selection of tests should be tailored to the patient's clinical presentation, duration of symptoms, travel history, medication use, and comorbidities to ensure appropriate diagnosis and management.
• A broader set of bacterial, viral, and parasitic agents should be considered in the context of a possible outbreak of diarrheal illness or in immunocompromised individuals 1. It is crucial to interpret the results of these tests in the context of the patient's clinical presentation and to consider the potential for false positives or false negatives, particularly with molecular tests 1.

From the Research

Diarrhea Evaluation Labs

To evaluate diarrhea, several labs can be ordered, depending on the suspected cause and duration of symptoms.

• For acute diarrhea, labs may include:
  • Stool tests for bacterial, viral, or protozoal infections 2
  • Glutamate dehydrogenase test for C. difficile infections 2
  • Specialized nucleic acid amplification test (for toxin B) or enzyme immunoassays for toxin A or B to distinguish infection from carrier state 2
• For chronic diarrhea, labs may include:
  • Tests for Giardia 2
  • Calprotectin 2
  • Fecal immunochemical test 2
  • Bile acid diarrhea test 2
  • Comprehensive stool biochemical analyses (osmolality, pH, electrolytes) to differentiate osmotic from secretory diarrhea and identify laxative abuse 2
  • Specific stool diagnostic tests for exocrine pancreatic insufficiency 2

Considerations for Lab Ordering

The decision to order labs for diarrhea evaluation should be based on the patient's symptoms, medical history, and circumstances of the illness.

• In cases where there is a strong pretest probability of infectious etiology or C. difficile infection, stool tests are indicated 2
• In resource-poor countries, the significance of diarrhea is even greater, and lab ordering should take into account the local epidemiology of diarrheal diseases 3