Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1326 page)

BORDETELLA PERTUSSIS
CULTURE (RULE OUT)

   Definition and Use

   This test is used to detect acute infection caused by the slow-growing, fastidious pathogen
B
.
pertussis
, the cause of pertussis or whooping cough. Nasopharyngeal specimens should be submitted for
B
.
pertussis
culture; aspirates are preferred. Anterior nares or throat specimens are unacceptable. Transport medium (e.g., Regan-Lowe) is recommended. Specimens are typically inoculated onto an enriched, selective agar, like Regan-Lowe.

   
Turnaround time:
Most cultures are positive in 7–10 days, although some cultures are incubated for up to 14 days. Additional time is required for final identification and further characterization.

   Interpretation

   
Expected results:
Negative. A negative culture does not exclude the diagnosis of pertussis, especially when a specimen is collected after the early, acute phase of infection.

   
Positive results:
Confirm the diagnosis of pertussis.

   Limitations

   The sensitivity of culture for
B
.
pertussis
falls significantly after the first 7–14 days after onset of symptoms. Poor specimen collection, submission of specimens other than nasopharyngeal specimens, and submission of specimens during the chronic phase of disease are associated with poor sensitivity of culture.

   Other Considerations

   PCR methods have been described for diagnosis of pertussis. Cross-reactions have been described (e.g.,
Bordetella holmesii
) and may limit the utility of molecular diagnostic testing. The sensitivity of PCR is greatest in the early acute phase of infection, but
B
.
pertussis
DNA may be detectable for weeks after resolution of acute disease. A number of serologic assays are commercially available, including assays for IgM and IgA. Variable sensitivity and specificity have limited the clinical utility of these assays.

BORDETELLA PERTUSSIS
SEROLOGY IgG

   Definition

   
Pertussis
is a respiratory tract infection caused by the gram-negative coccobacillus
Bordetella pertussis
. It is characterized clinically by a severe and prolonged cough. Coughing fits may be paroxysmal and, usually in infants, followed by an inspirational “whoop.” A clinical diagnosis will form the basis of most pertussis diagnosis and treatment decisions. The CDC provides the following clinical case definition for pertussis: A cough illness lasting at least 2 weeks with one of the following: paroxysms of coughing, inspiratory “whoop,” or post-tussive vomiting, without other apparent cause. This test is used to detect acute infection caused by the slow-growing, fastidious pathogen
B
.
pertussis
, the cause of pertussis or whooping cough. Because of the contagiousness of the infection, specific laboratory testing may be needed to confirm the diagnosis when pertussis is suspected clinically. Laboratory diagnosis of pertussis is complicated by the limitation of available tests. Options for diagnostic and confirmatory testing, when required, depend on the age of the patient and the phase of illness.