Gastroenteritis in Emergency Medicine Workup
- Author: Arthur Diskin, MD; Chief Editor: Steven C Dronen, MD, FAAEM more...
Laboratory Studies
- Determination of laboratory tests: The patient's evaluation should be based on the clinical assessment and the need to do the following:
- Further evaluate the seriousness of the condition (degree of dehydration and electrolyte derangement).
- Determine a specific etiologic agent.
- Evaluate the patient for noninfectious etiologies.
- Patients who require further workup include those who appear seriously ill or dehydrated; those who have high fevers, bloody stools, severe abdominal pain, or persistent diarrhea; and those who are immunocompromised or whose condition is suspected of having an epidemic diarrheal etiology.
- History, epidemiologic considerations, and the physical examination should be the primary guides in determining whether any further diagnostic evaluation is necessary, followed by microscopic examination of the stool.
- Stool studies and culture
- 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 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 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 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 PCR. This is usually reserved for epidemiological purposes. Rapid assays are becoming available with varying sensitivities/specificities and questionable clinical applicability.
- Routine laboratory tests
- Routine laboratory tests (CBC, electrolytes, renal function) may not be helpful or indicated in making a diagnosis. These tests may be useful as indicators of severity of disease, especially in elderly or very young patients, although that determination is best made clinically.
- Electrolytes and BUN tests are indicated in patients with severe diarrhea or dehydration to assess the state of hydration and to specifically rule out hyponatremia or hypernatremia. Decreased serum bicarbonate suggests severe dehydration, especially in children. Acidosis secondary to bicarbonate loss in the stools and/or from hypovolemia-induced lactic acidosis may be present. Hypokalemia may also occur.
- A CBC may be indicated with a prolonged course, severe diarrhea, or toxicity. The WBC count is usually increased in Salmonella infections but normal or low with significant left shift in Shigella infections. The WBC count is otherwise variable. Eosinophilia may be present in parasitic infections.
- Enzyme-linked immunosorbent assay (ELISA)
- Commercially available immunofluorescent antibody and enzyme immunoassays are also available for Giardia and Cryptosporidium organisms. C difficile toxin assays can be performed when antibiotic-associated diarrhea is suspected.
- Rotavirus: Enzyme-linked immunosorbent assay (ELISA) is available in less than 2 hours but is not sensitive enough in adults.
- Giardia: ELISA is more than 90% sensitive in susceptible populations (eg, people who camp or travel to endemic areas). Consider ELISA prior to ova and parasite examination or string test.
Imaging Studies
- An acute abdominal series is indicated only when bowel obstruction, toxic megacolon, or perforation is suspected.
- A low threshold for CT scanning should be maintained in post gastric bypass patients and older patients with significant abdominal pain.
Procedures
- Sigmoidoscopy may be indicated if pseudomembranous colitis or inflammatory bowel disease is suspected. Sigmoidoscopy is useful in obtaining tissue for culture in patients with AIDS who have undiagnosed diarrhea or wasting syndrome.
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