Approach Considerations
The gold standard for diagnosis of pertussis is isolation of B pertussis in culture. However, laboratory confirmation of pertussis is difficult and delayed. Therefore, clinicians need to make the diagnosis of pertussis presumptively in patients with a history of intense paroxysmal coughing with or without whooping, color changes, posttussive vomiting, incomplete or absent pertussis vaccination, and a finding of lymphocytosis on laboratory examination.
A clinical case of pertussis is defined as one of the following:
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An acute coughing illness that lasts at least 14 days in a person with at least one characteristic pertussis symptom (ie, paroxysmal cough, posttussive vomiting, or inspiratory whoop)
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A cough that lasts at least 14 days in an outbreak setting
A confirmed case is defined as one of the following:
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Any cough illness in which B pertussis is isolated and cultured
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A case consistent with the clinical case definition confirmed by polymerase chain reaction (PCR) assay findings or epidemiologic linkage to a laboratory-confirmed case
A photomicrograph of the bacterium Bordetella pertussis, using Gram stain technique.
Imaging studies typically add little to the diagnosis of pertussis but should be obtained when clinically indicated, based on examination or if the patient requires supplemental oxygen.
Serologic antibody titer testing is available, but often needs to be compared with results 1-2 weeks later and thus is not commonly helpful. The Centers for Disease Control and Prevention (CDC) also performs characterization of B pertussis isolates by serologic and molecular subtyping methods for outbreak support and other public health concerns. Laboratory testing is provided only upon prior communication with the Pertussis and Diphtheria Laboratory, indicating the reason for this service.
The use of direct fluorescent assay (DFA) of nasopharyngeal secretions is not recommended by the CDC; although the results can be available within minutes, the test has low sensitivity and specificity.
Chest radiography
Chest radiography may reveal perihilar infiltrates or edema with variable degrees of atelectasis. Consolidation is indicative of secondary bacterial infection or, rarely, pertussis pneumonia. Occasionally, pneumothorax, pneumomediastinum, or air in the soft tissues may be seen.
Blood Work
Leukocytosis (15,000-50,000/µL) with absolute lymphocytosis occurs during the late catarrhal and paroxysmal phases. It is a nonspecific finding but correlates with the severity of the disease. One study showed that among infants suspected of having pertussis, an absolute leukocyte count lower than 9400/μL excluded almost all infants who had a negative pertussis test finding. [29] In adults, especially those who have been vaccinated, lymphocytosis is rare.
In infants aged 90 days or younger, early serial monitoring of white blood cell (WBC) counts is crucial for identifying risk and determining the prognosis of infants with pertussis. A retrospective study of 31 infants with pertussis found that WBC counts higher than 30,000/μL (within a mean of 5.1 days after cough onset), rapid heart rates, and hyperventilation were indicators of severe B pertussis infection. [30, 31]
In this study, WBC counts in infants with severe disease tended to elevate more rapidly than those in infants with less severe disease. [30, 31] Moreover, WBC counts reached higher peaks in patients with severe pertussis than in those with less severe pertussis (mean, 74,200/μL vs 26,900/μL; median, 74,100/μL vs 24,200/μL).
Cultures
The results of blood culture are uniformly negative because B pertussis grows solely in the respiratory epithelium. The culture specimen should be obtained by using deep nasopharyngeal aspiration or by holding a flexible swab (Dacron or calcium alginate) in the patient's posterior nasopharynx for 15-30 seconds or until a cough is produced.
The sample special media (preferred media include Regan-Lowe or Bordet-Gengou agar and modified Stainer-Scholte media) should be promptly inoculated. B pertussis usually grows after 3-4 days; however, culture findings cannot be considered negative for pertussis until after 10 days.
Recovery rates are highest during the catarrhal or early paroxysmal phase and are low after the fourth week of illness.
See the following pertussis guideline pages from the CDC:
Culture findings may be negative in patients who were previously immunized, have received antimicrobial therapy, or have been coughing for more than 3 weeks. A negative culture finding does not exclude the diagnosis of pertussis.
PCR Assay and ELISA
PCR assays and antigen detection increasingly are used to assist in diagnosing pertussis. Advantages include greater sensitivity, more rapidly available results, and use later in the disease course or after antimicrobial therapy because the tests do not rely on the isolation of viable organisms. [32] Their use is limited by lack of standardization and incomplete understanding of the correlation between test results and the course of the illness.
A PCR assay may reveal 10 organisms per swab sample, and its sensitivity may be greater than that of culturing.
However, false-positive results have been a problem, with some reports of more than 50%. Although this or a positive culture is the case definition for reporting pertussis to the CDC or the World Health Organization (WHO), some now recommend confirmation with enzyme-linked immunosorbent assay (ELISA) before declaring an epidemic. (Many now consider serologic testing with ELISA to be the gold standard.)
The CDC recommends a combination of culture and PCR assay if a patient has a cough lasting longer than 3 weeks.
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A photomicrograph of the bacterium Bordetella pertussis, using Gram stain technique.
