Stool cultures are commonly done to identify bacteria associated with enteric infection. Of all specimens collected, feces are likely to contain the greatest number and variety of organisms. For a routine stool culture, the stool is examined to detect and to rule out Salmonella, Shigella, Campylobacter, Aeromonas, Plesiomonas, and predominating numbers of Staphylococcus organisms; cultures for yeast, Pseudomonas, Yersinia, Vibrio, and Shiga toxin–producing E. coli have to be specifically requested, depending on laboratory practice. Clostridium difficile causes antibiotic-associated colitis. It is diagnosed by detection of the toxins.

A single negative stool culture should not be considered the end point in testing. At least three stool cultures collected on separate days are recommended if the patient’s clinical picture suggests bacterial involvement despite previous negative cultures. Moreover, once a positive diagnosis has been made, the patient’s personal contacts should also be tested to prevent a potential spread of infection.

1. Procedure for stool specimen collection:

   1. Observe standard precautions.

   2. Use a dry container or a clean, dry bedpan to collect feces. Do not contaminate stool specimen with urine, water, soap, or disinfectants.

   3. Remember that a freshly passed stool is best. Diarrheal stool usually gives acceptable results.

   4. Select portions containing pus, blood, or mucus; 1–2 g is sufficient.

   5. Do not retrieve stool from the toilet for specimen use.

   6. Do not place toilet tissue or diapers with the specimen. Either may contain bismuth, which interferes with laboratory tests.

   7. Transfer stool specimens from the bedpan to the container with tongue blades.

   8. Label the sealed specimen container with the patient’s name, date, and tests ordered and immediately send it to the laboratory.

   9. Place the specimen in a transport medium, such as Cary-Blair transport medium, if a delay of longer than 2 hours for stool culture is anticipated (from time of collection until receipt in the laboratory). Specimens processed within 2 hours of collection do not require added preservatives.

2. Procedure for obtaining a rectal swab:

   1. Observe standard precautions.

   2. Insert the swab gently into the rectum (to a depth of at least 3 cm) and rotate it to retrieve a visible amount of fecal material (Figure 7.1).

   3. Place the swab into the receptacle containing transport medium, such as Cary-Blair transport medium.

   4. Label specimen with the patient’s name, date, and test(s) ordered. Send it in a biohazard bag to the laboratory as soon as possible.

   5. Rectal swab may not contain sufficient sample to detect enteric pathogens. Whenever possible, stool should be submitted.

3. Procedure for performing cellophane tape test for pinworm (Enterobius vermicularis):

   1. Observe standard precautions. The tape test is indicated in cases of suspected enterobiasis (pinworms).

   2. Apply a strip of clear cellophane tape (not micropore or adhesive-type tape) to the perineal region. Remove and spread the tape on a slide for microscopic examination.

   3. Remember that a paraffin-coated swab can be used in place of the cellophane tape test. If used, place the swab within a stoppered test tube.

   4. Be aware that it may be necessary to make four to six examinations on consecutive days before ruling out the presence of pinworms.

   5. Test for pinworm eggs in the morning before the patient has defecated or bathed.

1. C. albicans, S. aureus, and P. aeruginosa, found in large numbers in the stool, are considered pathogenic in the setting of previous antibiotic therapy. Alterations of normal flora by antibiotic drugs often change the environment so that normally harmless organisms become pathogens.

2. Cryptosporidiosis is a cause of severe, protracted diarrhea in immunosuppressed patients. Cryptosporidium organisms can be detected by ova and parasite examination.

3. Helicobacter pylori has been associated with gastritis and peptic ulcer disease. H. pylori is found only on the mucus-secreting epithelial cells of the stomach. Detection of H. pylori in gastric biopsy specimens necessitates collection of the specimens in sterile containers. Smears and cultures should be examined for the presence of this organism. Initial culture incubation requires 7 days. Therefore, results of gastric biopsy specimen cultures may take 8–10 days to obtain. A test for H. pylori antigen in the stool provides rapid detection of H. pylori.

4. C. difficile: Whenever normal flora are reduced by antibiotic therapy or other host factors, the syndrome known as pseudomembranous colitis can occur. This condition is caused by C. difficile. It may be present in small numbers in the normal person, or it may occur in the hospital environment. When normal flora are reduced, C. difficile can multiply and produce its toxins.

The definitive diagnosis of C. difficile–associated diarrhea is based on clinical criteria. Endoscopic visualization of a characteristic pseudomembrane or plaque, together with a history of antibiotic therapy, is diagnostic of C. difficile. Three laboratory tests are also available. These include stool culture for C. difficile (nonspecific; requires at least 48 hours); tissue culture for detection of cytotoxin (requires 48 hours); and rapid tests that are sensitive and specific for C. difficile antigens and toxins.

Pretest Patient Care

1. Explain purpose of test, procedure for stool collection, and interfering factors. Assess for and document history of diarrhea, including type and length of time. Instruct the patient to defecate into a clean, dry bedpan or large-mouthed container.

2. Do not allow patient to defecate into the toilet bowl or urinate into the bedpan or collecting container because urine has an adverse effect on protozoa.

3. Do not place toilet paper into the bedpan or collection container; it may contain bismuth, which can interfere with testing.

4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Stool from patients receiving barium, bismuth, mineral oil, or antibiotic drug is not satisfactorily a specimen for identifying protozoa.

Normal

The following organisms may be present in the stool of apparently healthy people:

1. C. albicans

2. Enterococcus spp.

3. E. coli

4. Proteus spp.

5. P. aeruginosa

6. Streptococcus spp.

7. Staphylococcus spp.