Bordetella pertussis - Whooping Cough

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Contributor

Couturier

Professor of Pathology (Clinical), University of Utah

Head of Clinical Operations for Clinical Microbiology and Immunology; Medical Director, Emerging Public Health Crises, Parasitology/Fecal Testing, and Infectious Disease Antigen Testing, ARUP Laboratories

Pertussis, also known as whooping cough, is an acute infectious disease caused by the Bordetella pertussis bacterium. In babies and young children, pertussis is especially dangerous and can result in potentially deadly complications, such as pneumonia, apnea, and encephalopathy. The CDC recommends vaccination against whooping cough for young children, preteens, pregnant women, and unvaccinated adults. The protective power of a pertussis vaccine fades over time, so being up to date with vaccination is important, particularly for families and caregivers of new babies. The early diagnosis and treatment of pertussis are extremely important to limit disease spread. Laboratory tests such as polymerase chain reaction (PCR), culture, and serology can detect the presence of the B. pertussis bacterium. These tests are highly dependent on the time that has elapsed since initial infection, so the testing strategy should be developed in reference to the time of disease onset. The CDC-recommended laboratory tests for pertussis diagnosis are blood culture and PCR.^, Additionally, although serologic testing is not confirmatory for the purpose of reporting, it may help to identify late-stage pertussis.^,

Quick Answers for Clinicians

What is the importance of laboratory testing for the diagnosis and surveillance of pertussis?

Early diagnosis and treatment of pertussis (whooping cough) limit its spread to other susceptible people. According to the most recent case definition for pertussis, a positive culture or polymerase chain reaction (PCR) laboratory test result is a required criterion for diagnosis. Laboratory testing enables the collection of accurate surveillance data to assess the impact of pertussis and monitor epidemiologic changes over time. The CDC recommends testing for patients with both high and low suspicion of pertussis for this reason. It is important to note that clinicians should not delay prophylactic treatment for close contacts, particularly infants, pregnant women, and individuals with preexisting health conditions, pending laboratory test results.

Which sample types are appropriate for polymerase chain reaction and serologic testing for pertussis diagnosis?

A posterior nasopharyngeal (NP) swab or aspirate should be obtained from all persons with a suspected case of pertussis (whooping cough). A posterior NP swab is the preferred sample type for adults and adolescents, whereas aspirates are preferable for testing in neonates, infants, and young children. A properly obtained NP swab or aspirate is essential for optimal results. Throat swabs and anterior nasal swabs have an unacceptably low rate of DNA and bacteria recovery and should not be used for pertussis diagnosis. The CDC provides detailed information and instructional videos on best practices for specimen collection on its website.

Where can I find more information about pertussis?

The Council of State and Territorial Epidemiologists (CSTE) published an approved case definition for pertussis (whooping cough) in position statement 19-ID-08. The CDC also provides this definition on its website.

There are many other useful, up-to-date CDC resources for clinicians and patients. Resources of note include vaccination information for pregnant women.

Indications for Testing

Patients who have a cough lasting ≥2 weeks with one or more of the following signs or symptoms should be tested for pertussis^,:

Paroxysms of cough
Inspiratory whoop
Posttussive vomiting
Apnea with or without cyanosis

Additionally, given that paroxysmal cough with an absence of fever is closely linked to pertussis, the absence of a fever may further indicate the need for testing.

However, because early diagnosis and treatment of pertussis might limit its spread, and laboratory testing methods are highly time dependent (Optimal Timing for Pertussis Laboratory Testing), it may be appropriate to begin testing sooner than 2 weeks from cough onset if pertussis is strongly suspected.

Laboratory Testing

Laboratory testing is extremely important for the diagnosis and surveillance of pertussis. However, when pertussis is strongly suspected, prophylaxis should be provided to household and other close contacts at high risk without waiting for laboratory confirmation.

The sensitivity of PCR, culture, and serologic testing is heavily impacted by the time from disease onset. Therefore, selection of appropriate laboratory testing strategies is time dependent. The figure below depicts the optimal timing for PCR, culture, and serologic testing for pertussis diagnosis.

CDC-adapted graphic detailing optimal sample collection times for pertussis PCR, culture, and serology testing.
Figure adapted from CDC

PCR

PCR is a highly sensitive CDC-recommended laboratory test that is especially useful for the timely diagnosis of pertussis. Patients with signs and symptoms consistent with pertussis should be tested by PCR to confirm the diagnosis. Specimens for PCR testing should be obtained by aspiration or by swabbing the posterior nasopharynx. Unlike culture, PCR testing does not require live bacteria for accurate results. The optimal time to collect a sample for PCR testing is in the first 3 weeks of illness, while bacterial DNA is still present in the nasopharynx. After the fourth week of illness, the amount of bacteria in the nasopharynx diminishes quickly, which increases the risk of a false-negative result. Although PCR is a powerful test due to its high sensitivity, the CDC recommends that it be ordered in conjunction with culture when feasible. PCR testing is not recommended for asymptomatic patients because false-positive results are more likely in these individuals.

Culture

Bacterial culture is a highly specific CDC-recommended laboratory test and is considered the gold standard for pertussis diagnosis because it enables strain identification and antimicrobial resistance testing. Patients with suspected pertussis should have a nasopharyngeal (NP) swab or aspirate obtained from the posterior nasopharynx to confirm the diagnosis.^, The optimal time to collect a culture specimen is during the first 2 weeks of illness because this is when viable bacteria are present in the nasopharynx. Culture specimens taken after 2 weeks have low specificity and are more likely to produce false-negative results. If the patient has received antibiotic therapy or previous vaccination against B. pertussis, the sensitivity of bacterial culture decreases. Physicians should consider concurrent PCR testing.

Serology

Serologic testing is not recommended by the CDC for pertussis; however, it is often useful for the diagnosis of late-stage pertussis. The optimal time to collect a sample for serologic testing is 2-8 weeks after cough onset. This is when the antibody titers are highest. However, serologic testing can be performed on samples collected up to 12 weeks after the onset of illness.

Several factors can contribute to inaccurate serology results. These include recent vaccination, previous infection, and cross-reactivity with other Bordetella species. Serologic testing is often ineffective in infants younger than 6 months due to interference from maternal antibodies. As of March 2017, positive serology results from commercial laboratories were no longer considered confirmatory for the purpose of reporting; for that reason, serology is not a CDC-recommended test for pertussis diagnosis.

Serologic testing can be used to determine immune status. Please see the ARUP Consult Immunization Status topic for more information.

Comparison of Pertussis Testing Methodologies

Test Methodology	Advantages	Disadvantages
PCR	CDC-recommended test High sensitivity Does not require live bacteria for accurate result	Must be performed during first 3 wks of illness for accurate results Specificity varies Increased risk of false positivity
Culture	CDC-recommended test High specificity Enables strain identification and antimicrobial resistance testing	Must be performed during first 2 wks of illness for accurate results Low sensitivity Requires live bacteria
Serology	Can be performed up to 12 wks after disease onset Can determine previous exposure to B. pertussis	Not a CDC-recommended test Recent vaccination, previous infection, and cross-reactivity with other Bordetella species may cause inaccurate results Is often ineffective in infants younger than 6 mos
Sources: CDC^,^,; Van der Zee, 2015; Nieves, 2016