Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (146 page)

   
Trichinella spiralis

   
Vibrio vulnificus

   Diagnosis and Reporting

Most cases of diarrheal illness are mild and self-limited, and testing to establish a specific cause is rarely necessary. Diagnostic testing is recommended for patients with profuse, watery diarrhea, passage of stools with blood or mucus, persistent diarrhea (>48 hours), immunocompromised patients, and patients with severe gastrointestinal or systemic symptoms (like severe abdominal pain, fever, or hypovolemia). Testing to establish a specific diagnosis is also recommended for patients at risk for complications of gastrointestinal infections, like patients with inflammatory bowel disease, patients involved in any investigation of a possible outbreak of diarrheal illness, and patients who may be at increased risk for transmitting infection to others, like food handlers.

Because of the diverse etiology and variety of tests required to make a specific diagnosis, consultation with infectious disease experts and clinical microbiologists may improve diagnostic strategies. For many agents of foodborne diseases, reporting to the local department of public health is required; public health officials may be able to provide important information concerning ongoing outbreaks or diagnostic support.

Diagnostic testing
: The type of testing will depend on the agent suspected, clinical presentation, source of specimen submitted for testing and other factors. Diagnostic techniques for microbial pathogens are discussed in other sections of this book.

   
Bacteria
: Bacterial pathogens may be isolated by culture of stool, vomitus, or other patient specimen. Stool culture is most commonly submitted. Consider submitting stool for culture in patients with

   Immunocompromise or increased risk for complication of bacterial gastroenteritis

   Inflammatory bowel disease, to distinguish between infection and flare

   Severe illness, including severe vomiting or diarrhea, abdominal pain, hypovolemia, or prolonged duration

   Signs of inflammation, like blood, mucus, or leukocytes in stool; fever; or sepsis; involvement of organ systems outside the gastrointestinal tract

Fecal culture requires use of selective, differential media optimized for isolation of specific pathogens. The pathogens routinely tested may vary from laboratory to laboratory.
Campylobacter
,
Salmonella,
and
Shigella
spp. are typically isolated by routine stool cultures. Antigen assays are also available for sensitive and specific direct detection of
Campylobacter
in stool specimens. Special cultures may have to be requested if another pathogen is suspected on clinical or epidemiologic grounds (e.g.,
E. coli
O157:H7, other Shiga-toxigenic enteric GNB,
Vibrio
spp.,
Aeromonas
spp.,
Listeria monocytogenes
). Bacterial pathogens are present in high concentrations during acute symptomatic infection. Therefore, submission of a single specimen is usually sensitive for detection of bacterial causes of diarrhea; repeat cultures may be necessary for detection of
Shigella
or asymptomatic carriage of an enteric pathogen.

After isolation, additional testing may be required to detect specific pathogenic mechanisms (e.g., enteropathogenic
E. coli
, Shiga toxin production) or for epidemiologic studies (e.g., serotype analysis of
Salmonella
isolates). Culture of blood, CSF, and other specimens are recommended for patients with signs of systemic illness or localized infection outside the gastrointestinal tract.

In hospitalized patients with onset of symptoms more than 48 hours after admission, testing for
Clostridium difficile
is recommended; routine stool culture or O&P evaluation is unlikely to yield clinically significant results. Several different assays may be used to detect toxigenic
C. difficile
, including toxin A and B EIA, specific glutamate dehydrogenase, cytotoxicity, isolation by anaerobic bacterial culture and PCR.

Evaluation of food or environmental specimens for enteric pathogens is typically performed by a Public Health Laboratory, or other specialized reference laboratory, and is usually performed only as part of a formal epidemiologic investigation.

   
Enteric viruses
: Viral gastroenteritis is most commonly mild and self-limited with few systemic symptoms and may be effectively treated symptomatically without establishing a specific diagnosis. Four viral pathogens are responsible for most cases of viral gastroenteritis in the United States: norovirus, rotavirus, enteric adenoviruses, and astroviruses.

   
Viruses may be
detected in stool by electron microscopic techniques, but this testing is not available for routine evaluation of patient specimens.