Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1337 page)

   
Common pitfalls:

   Poor specimen collection (sample selection or collection technique) or loss of viability during transport. Swabs may be toxic for
C
.
trachomatis
. Specific types and lots of swabs should be tested for toxicity before releasing before clinical use. Urethral specimens should not be collected within 1 hour after the patient has urinated.

CLOSTRIDIUM DIFFICILE
DETECTION

   Definition and Use

   
Clostridium difficile
is a major cause of antibiotic-associated diarrhea and pseudomembranous colitis, and it represents a significant and serious agent of nosocomial infection.
C
.
difficile
infection (CDI) may be mild and selflimited after discontinuation of antibiotics, but a significant number of patients have persistent and/or severe diarrheal illness that may progress to pseudomembranous colitis or toxic megacolon.
C
.
difficile
is an anaerobic, spore-forming, gram-positive bacillus. It forms several toxins (toxins A and B) that have been used as the basis for detection.
C
.
difficile
detection is recommended for patients in whom diarrheal illness develops after antibiotic therapy or during hospitalization, especially when colitis (increased fecal leukocytes) is a prominent feature.

   
Methods

   
Cytotoxicity:
In this method, a suspension of stool, passed through a membrane filter to remove bacteria, is inoculated on cultured eukaryotic cells (e.g., WI-38). The cell monolayer is examined for 48 hours for evidence of typical “actinomorphic” cytotoxicity (disruption of the normal cellular morphology in the monolayer). In order to exclude nonspecific cytotoxicity that may be seen with stool filtrates, neutralization of the cytotoxic effect, using anti–
C
.
difficile
antiserum, must be demonstrated.

   
Toxigenic culture:
C
.
difficile
may be isolated from stool using a spore selection technique (by heat shock or alcohol pretreatment of a stool suspension prior to medium inoculation) and selective media. Because not all
C
.
difficile
isolates produce the toxins associated with disease, culture supernatants must be tested for toxin production to make a diagnosis of
C
.
difficile
–associated disease.

   
Toxin detection using immunodiagnostic procedures:
A number of commercially available latex agglutination or EIA kits have been developed for the detection of
C
.
difficil
e toxin A and/or toxin B in stool samples. These assays provide a more rapid turnaround time, but lower sensitivity and specificity, compared to culture or cytotoxicity assays.

   
C
.
difficile
glutamate dehydrogenase antigen detection:
Detection of
C
.
difficile
glutamate dehydrogenase antigen may be used to screen stool for the presence of
C
.
difficile
. Because glutamate dehydrogenase is not specific for toxigenic
C
.
difficile
, positive specimens must be tested for toxin to confirm the diagnosis of
C
.
difficile
–associated disease.

   
Molecular diagnostic methods:
Molecular diagnostic methods for detection of the
C
.
difficile
are commercially and provide the most sensitive assay for CDI. The test is very specific when used on patients with typical clinical signs and symptoms of
C
.
difficile
disease.