LabMed

Large Intestine, Infectious Diarrhea: Outpatient, Including Traveler’s Diarrhea

At a Glance

Diarrhea is suspicious for an infectious etiology.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The following tests should be ordered:

  1. Stool culture for Salmonella, Shigella, Campylobacter, Yersinia, and Vibrio (may have to order separately)

  2. If available, consider multiplex gastrointestinal PCR panel (includes everything below plus norovirus, other viruses, as well as toxigenic E. coli). Exact panel depends on the manufacturer - but be aware charges may be up to $500 - 1000/test for outpatients.

  3. Giardia and Cryptosporidia antigen

  4. Fecal Lactoferrin (better test than fecal leukocytes)

  5. Microscopic exam for ova and parasites

  6. Shiga toxin for E. coli O157 and other toxin producing strains

  7. C. difficile toxin A/B, PCR, or C. difficile GDH antigen (only if the patient has taken antibiotics or has a history of C. difficile)

(Table 1)

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

For all tests, the specimen should be transported to the lab as soon as possible but can be stored refrigerated for 24-48 hours if necessary.

Stool for O and P should be collected in preservative, so routine transport to the lab is acceptable.

Barium will interfere with most stool tests. Mineral oil may interfere with the antigen tests.

Preparation for endoscopy may make stool testing falsely negative because of extreme dilution, although biopsies are fine.

Antibiotic treatment will reduce the number of bacterial pathogens in stool and lower the sensitivity of bacterial stool culture and Shiga toxin. Flagyl (metronidazole) and oral vancomycin will lower test sensitivity for C. difficile, and Flagyl will also interfere with detection of Giardia.

Since improvement is the natural history for many, if not most, cases of infectious diarrhea, especially the less severe cases, the longer the time from onset of symptoms, the less likely a test is to be diagnostic.

What Lab Results Are Absolutely Confirmatory?

A positive result from any of the tests listed is considered diagnostic, and confirmatory testing is not needed.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Further confirmatory testing of positive results is not necessary. If there is symptomatic improvement, only rarely is there a need to document that a positive test has become negative.

Mildly ill patients should be tested stepwise (i.e., test for Giardia and Cryptosporidia with or without stool culture and watchful waiting). If symptoms persist, microscopic stool exam for Ova (O) and Parasites (P) and fecal lactoferrin can then be done. Because parasites can be shed intermittently, three separate stool specimens are recommended for O and P. Alternatively, the recently available multiplex gastroenteritis PCR panels will test for almost all known infectious agents of diarrhea and can be done within 24 hours, depending on the laboratory. Although convenient, patients may receive a very large bill.

The large majority of cases of traveler’s diarrhea is due to toxin producing and enteroinvasive strains of E. coli not detected with current laboratory diagnostic methods, so that most cases will have no etiology proven. Patients generally respond to empiric treatment or resolve spontaneously, but about 5-10% may have lingering symptoms typical of irritable bowel syndrome that can last for months to years.

If diarrhea persists after routine studies and multiplex gastrointestinal PCR testing are negative, further studies depend on the severity of illness. If the lactoferrin test is positive, noninfectious inflammatory bowel disease must be considered.

If the patient is HIV positive, CMV is an important consideration that would not be the case in patients with normal immunity; in HIV patients, CMV is the leading cause of bloody diarrhea. Colonoscopy is required for the diagnosis of CMV colitis, since direct tests for CMV in stool have not been developed.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

As with testing, empiric treatment with antibiotics, antiparasitic or antiviral drugs (e.g., Ganciclovir for CMV in HIV) will greatly lower the sensitivity of laboratory testing for the respective infectious agents.

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