Diarrhea Workup

Updated: Jan 31, 2020
  • Author: Stefano Guandalini, MD, AGAF; Chief Editor: Carmen Cuffari, MD  more...
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Laboratory Studies

The following may be noted in patients with diarrhea:

  • In patients with diarrhea, a stool pH level of 5.5 or less or presence of reducing substances indicates carbohydrate intolerance, which is usually secondary to viral illness and transient in nature.

  • Enteroinvasive infections of the large bowel cause leukocytes, predominantly neutrophils, to be shed into stool. Absence of fecal leukocytes does not eliminate the possibility of enteroinvasive organisms. However, presence of fecal leukocytes eliminates consideration of enterotoxigenic E coli, Vibrio species, and viruses.

  • Examine any exudates found in stool for leukocytes. Such exudates highly suggest colitis (80% positive predictive value). Colitis can be infectious, allergic, or part of inflammatory bowel disease (Crohn disease, ulcerative colitis).

  • Many different culture mediums are used to isolate bacteria. Table 3 lists common bacteria and optimum culture mediums for their growth. A high index of suspicion is needed to choose the appropriate medium.

  • With stool not cultured within 2 hours of collection, refrigerate at 4°C or place in a transport medium. Although stool cultures are useful when positive, yield is low.

  • Always culture stool for Salmonella, Shigella, and Campylobacter organisms and Y enterocolitica in the presence of clinical signs of colitis or if fecal leucocytes are found.

  • Look for C difficile in persons with episodes of diarrhea characterized by colitis and/or blood in the stools. Remember that acute-onset diarrheal episodes associated with C difficile may also occur without a history of antibiotic use.

  • Bloody diarrhea with a history of ground beef ingestion must raise suspicion for enterohemorrhagic E coli. If E coli is found in the stool, determine if the type of E coli is O157:H7. This type of E coli is the most common, but not only, cause of HUS.

  • History of raw seafood ingestion or foreign travel should prompt additional screening for Vibrio and Plesiomonas species.

Table 4. Common Bacteria and Optimum Culture Mediums (Open Table in a new window)


Detection Method

Microbiologic Characteristics

Aeromonas species

Blood agar

Oxidase-positive flagellated gram-negative bacillus (GNB)

Campylobacter species

Skirrow agar

Rapidly motile curved gram-negative rod (GNR); Campylobacter jejuni 90% and Campylobacter coli 5% of infections

C difficile

Cycloserine-cefoxitin-fructose-egg (CCFE) agar; enzyme immunoassay (EIA) for toxin; latex agglutination (LA) for protein

Anaerobic spore-forming gram-positive rod (GPR); toxin-mediated diarrhea; produces pseudomembranous colitis

C perfringens

None available

Anaerobic spore-forming GPR; toxin-mediated diarrhea

E coli

MacConkey eosin-methylene blue (EMB) or Sorbitol-MacConkey (SM) agar

Lactose-producing GNR

Plesiomonas species

Blood agar

Oxidase-positive GNR

Salmonella species

Blood, MacConkey EMB, xylose-lysine-deoxycholate (XLD), or Hektoen enteric (HE) agar

Nonlactose non–H2S-producing GNR

See the list below:

  • Culture mediums used to isolate bacteria include the following:

    • Blood agar - All aerobic bacteria and yeast; detects cytochrome oxidase production

    • MacConkey EMB agar - Inhibits gram-positive organisms; permits lactose fermentation

    • XLD agar; HE agar - Inhibits gram-positive organisms and nonpathogenic GNB; permits lactose fermentation H2S production

    • Skirrow agar - Selective for Campylobacter species

    • SM agar - Selective for enterohemorrhagic E coli

    • CIN agar - Selective for Y enterocolitica

    • TCBS agar - Selective for Vibrio species

    • CCFE agar - Selective for C difficile

  • Rotavirus antigen can be identified by enzyme immunoassay and latex agglutination assay of the stool. The false-negative rate is approximately 50%, and false-positive results occur, particularly in the presence of blood in the stools.

  • Adenovirus antigens can be detected by enzyme immunoassay. Only serotypes 40 and 41 are able to induce diarrhea.

  • Examination of stools for ova and parasites is best for finding parasites. Perform stool examination every 3 days or every other day.

  • The leukocyte count is usually not elevated in viral-mediated and toxin-mediated diarrhea. Leukocytosis is often but not constantly observed with enteroinvasive bacteria. Shigella organisms cause a marked bandemia with a variable total white blood cell count.

  • At times, a protein-losing enteropathy can be found in patients with extensive inflammation in the course of enteroinvasive intestinal infections (eg, Salmonella species, enteroinvasive E coli). In these circumstances, low serum albumin levels and high fecal alpha1-antitrypsin levels can be found.


Other Tests

Because the pathogenesis of diarrhea can be either osmolar (due to the presence of an excess of unabsorbed substrates in the gut lumen) or secretory (due to active anion secretion from the enterocytes), the anion gap in the stools is occasionally used to ascertain the nature of the diarrhea. The stool anion gap is calculated according to the formula: 290 - [(Na+K) X 2]. If the value is more than 100, osmolar diarrhea can be assumed to be present. If the value is less than 100, the diarrhea has a secretory origin.



Intestinal biopsy is not required in evaluating an otherwise healthy child with acute-onset diarrhea, but it may be indicated in the presence of chronic or protracted diarrhea, as well as in cases in which a search for a cause is believed to be mandatory (eg, in patients with acquired immunodeficiency syndrome [AIDS] or patients who are otherwise severely immunocompromised).