Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (573 page)

   Who Should Be Suspected/Who Should Be Tested

   The most important issue in pertussis diagnosis is clinical recognition. Typical cases of pertussis demonstrate three phases:

   Catarrhal (7–10 days): Runny nose; mild cough; low-grade fever. The burden of
B. pertussis
is highest in the catarrhal phase.

   Paroxysmal (1–6 weeks): Severe, paroxysmal coughing spells; inspiratory whoop; cyanosis; posttussive vomiting.

   Recovery (2–4 weeks): Decreasing severity of symptoms.

Infants and patients who are unvaccinated, are immunocompromised, or have underlying medical conditions are more likely to have more severe symptoms.

   Clinical case definition: Recognition of the pertussis syndrome is critical for diagnosis. A clinical case is defined as a cough illness lasting at least 2 weeks (without other cause), with at least one of the following features: paroxysms of coughing, inspiratory “whoop” (most common in infants), posttussive vomiting.

   In the context of a pertussis epidemic, any patient with a prolonged cough illness, regardless of other symptoms, may be suspected.

   Diagnostic and Laboratory Findings

A number of test methods are available for detection of
B. pertussis
infection.

Tests recognized by the CDC for confirmation of pertussis
:

Culture
: Culture should be obtained from all suspected cases of pertussis. Isolation of
B. pertussis
confirms the diagnosis (specificity: approximately 100%), but cultures are frequently negative (sensitivity: 15–35%). Nasopharyngeal aspirate or swab (not cotton) specimens should be collected in the first 2 weeks of illness. Specimens should be directly plated onto supportive media, like Regan-Lowe or Bordet-Gengou, or inoculated into supportive media, like half-strength Regan-Lowe charcoal–blood media, for immediate transport to the laboratory. Negative cultures may be due to a number of factors including collection of specimen >2 weeks after the onset of illness, improper collection (e.g., site, swab type), delayed or improper transport conditions, prior antibiotic therapy, and recent vaccination.

PCR
: Though there is no FDA-approved test for
B. pertussis
, PCR is playing an increasingly important role in diagnosis. Testing performed by a Public Health Laboratory is recommended. PCR methods have high sensitivity (93–95%) and specificity (97–99%) when performed on appropriate patients. PCR testing should only be performed on patients with a clinical diagnosis of pertussis. PCR should not be performed on asymptomatic contacts or other asymptomatic patients. Nasopharyngeal aspirate or swab (Dacron, rayon, or nylon) should be collected within the first 3 weeks after the onset of illness. Antibiotic therapy may result in false-negative PCR results.

Tests not recognized by the CDC for confirmation of pertussis
:

DFA
: Though very specific (>95%), the sensitivity of DFA, compared to PCR, is low (10–50%). For initial clinical management, if timely PCR testing is not available, DFA may be considered.

Serology
: Serologic testing has limited utility for the diagnosis or management of patients with suspected pertussis. Serologic responses in patients usually occurs 2 or more weeks after the onset of cough (after the time when antibiotic therapy may be useful). Performance characteristics of commercially available tests have not been well defined for diagnosis of pertussis, and Public Health officials do not accept them for confirmation of pertussis. However, a single-point test has been validated by the Massachusetts Public Health Laboratory and is accepted by the CDC for confirmation of pertussis. This assay cannot be used for vaccinated children <11 years old or in adults vaccinated within 2 years.

   Interpretation of Test Results

Confirmed
:

   Clinical: Any cough illness; lab: isolation of
B. pertussis
by culture

   Clinical: Meets CDC Clinical Case Definition; lab: positive PCR for
B. pertussis