LabMed

Large Intestine, Infectious Diarrhea: Inpatient

At a Glance

Diarrhea in an inpatient 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. C. difficile - by PCR if available; otherwise toxin A/B or C, difficile GDH antigen with toxin A/B

  2. If available, multiplex gastrointestinal PCR panel (includes everything below plus norovirus, rotavirus, and a variety of toxigenic E. coli species depending on the manufacturer)

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

  4. Giardia and Cryptosporidia antigen

  5. Fecal Lactoferrin (better test than fecal leukocytes)

  6. Microscopic exam for Ova and Parasites (O&P)

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

(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?

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 the stool culture, Giardia antigen, Shiga toxin, and C. difficile tests. Note that only Flagyl (metronidazole) and oral vancomycin will lower the test sensitivity, because they specifically treat C. difficile colitis; treatment with other antibiotics actually induces or promotes C difficile colitis and, therefore, increases the pretest probability of a positive result.

Testing patients who develop diarrhea after 3 days in the hospital should be limited to C. difficile toxin testing; bacterial stool pathogens, ova, and parasites should be tested on admission when the patient first presents with diarrhea. Many labs will reject stool specimens for these tests because of the low probability of a positive result. This approach may change with the introduction of the multiplex GI panels; since they may detect so many unanticipated pathogens, there are both infection control and potential treatment options.

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 generally not necessary.

Follow-up testing of patients positive for C. difficile does not need to be done for at least 1 week, since neither a positive or a negative result within this time frame will change treatment. If the patient initially tested negative and diarrhea persists, C. difficile testing can be repeated after 48-72 hours, since the yield of repeat testing of an initially negative test (either toxin or polymerase chain reaction (PCR) assays) within this time period is extremely low.

In immunosuppressed patients (i.e., those who have had transplant or chemotherapy or have HIV), especially with bloody diarrhea, CMV colitis is a very important consideration. Endoscopy is required to make this diagnosis. CMV PCR in blood does not correlate with gastrointestinal CMV disease.

In patients who are extremely ill, laboratory testing should be performed as soon as possible and colonoscopy may need to be considered early, even before laboratory results are back. Empiric treatment should be aimed at the likely causes depending on the patient’s underlying conditions and diagnoses. Gastrointestinal (GI) and ID consultants should be involved early on.

If no infectious agents are found and if the fecal lactoferrin is positive, noninfectious inflammatory bowel disease should be considered.

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 or 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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