What is the role of stool studies and culture in the workup of gastroenteritis?

Updated: Feb 10, 2017
  • Author: Arthur Diskin, MD; Chief Editor: Steven C Dronen, MD, FAAEM  more...
  • Print

Note the following:

  • The presence of blood or leukocytes in stool is a strong indicator of inflammatory diarrhea.

  • Stool studies can be performed efficiently and inexpensively by using a Wright stain or methylene blue and directly observing for leukocytes and performing an occult blood test.

  • Fecal leukocytes are present in 80-90% of all patients with Salmonella or Shigella infections but are less common with other infecting organisms such as Campylobacter and Yersinia. They may also be present in ulcerative colitis and Crohn disease but are usually absent in viral infections, Giardia infection, enterogenic E coli infection, and toxigenic bacterial food poisoning.

  • A stool culture is not necessary or cost-effective in most cases of diarrhea unless an unusual bacterial cause is suspected and it may be needed for epidemiological purposes.

  • A lower threshold for performing stool cultures and examination for ova and parasites is indicated in immunocompromised, immunosuppressed patients and those who have recently traveled to remote locations or developing nations.

  • Fever, bloody stools, leukocytes in stool, pain resembling that associated with appendicitis (Yersinia), and diarrheal illness associated with partially cooked hamburger (cytotoxigenic E coli O157:H7) are all indications for culture. If possible, the laboratory should be informed of suspected organisms.

  • Frequently, stool cultures are obtained inappropriately in the United States. Consider whether obtaining a culture would change the therapy.

  • Specific indications for stool cultures include bloody stools, stools that test positive for occult blood or leukocytes, prolonged course of diarrhea that has not been treated with antibiotics, immunocompromised host, or for epidemiologic purposes, such as cases involving food handlers.

  • Routine stool cultures identify only Campylobacter, Shigella, Salmonella, Aeromonas, and Yersinia species.

  • Testing for other pathogens, such as Vibrio species, enterohemorrhagic E coli O157:H7, and other Shigatoxin-producing bacteria require special media. The laboratory should be informed regarding the need for appropriate media for suspected organisms (eg, MacConkey sorbitol agar for E coli O157:H7). Additionally, the laboratory may need to perform specialized testing to specifically identify the organism.

    The MacConkey medium is commonly used and differen The MacConkey medium is commonly used and differentiates lactose fermenters, which produce acid, decrease the pH, and cause the neutral red indicator to give the colonies a pink-to-red color.
    Hektoen enteric agar with Escherichia coli colonie Hektoen enteric agar with Escherichia coli colonies. Different growth media are necessary for identifying different enteric pathogens, suppressing the growth of nonpathogens, and allowing for chemical reactions to assist in identification. The appearance results from the organism's ability to ferment lactose placed in the medium. This results in the production of acid, which lowers the pH and causes a change in the pH indicator placed in the medium. Salmonella and Shigella organisms do not ferment lactose.
    Example of Salmonella on Hektoen enteric agar. The Example of Salmonella on Hektoen enteric agar. The medium also contains ferric ammonium citrate, which indicates the production of hydrogen sulfide by the appearance of a black precipitate.
  • Studies of selected centers have shown that only 2% of stool culture results are positive as routinely obtained. The cost per positive stool culture result has been estimated to be at least $900-1200.

  • Similarly, if parasitic illness is in the differential or if the patient has recently traveled to an endemic region or has chronic diarrhea, the stool should be examined for parasites or their ova with the caveat that several samples may be required to make the diagnosis. Ova and parasite studies are indicated for patients who are immunocompromised, who have a persistent or prolonged course, or whose conditions are unresponsive to antibiotics.

  • Travel to endemic regions followed by chronic diarrhea without signs of acute bacterial diarrhea should prompt a search for a parasitic etiology.

  • Entamoeba histolytica can result in bloody stools, but a smear reveals a lack of leukocytes due to exotoxin produced by the parasite that lyses the cells.

  • Stool can be sent to reference labs for examination for norovirus by polymerase chain reaction (PCR). This is usually reserved for epidemiological purposes. Rapid assays are becoming available with varying sensitivities/specificities and questionable clinical applicability.

Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!