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 (in infants younger than 1 year)
However, because early diagnosis and treatment of pertussis might limit its spread, and laboratory testing methods are highly time dependent (see below), it may be appropriate to begin testing in cases of strong suspicion of pertussis earlier than 2 weeks from cough onset.
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 serology 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 serology testing for pertussis diagnosis.
Figure adapted from CDC
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.
Bacterial culture is a highly specific CDC-recommended laboratory test and is considered the gold standard for pertussis diagnosis because it allows for strain identification and antimicrobial resistance testing. Patients with suspected cases of pertussis should have a nasopharyngeal 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 is not a CDC-recommended test; however, it is often useful for the diagnosis of late-stage pertussis. The optimal time to collect a sample for serology is 2-8 weeks following cough onset. This is when the antibody titers are highest. However, serology 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. Serology 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.
Serology can be used to determine immune status. Please see the Immunization Status topic for more information.
Comparison of Pertussis Testing Methodologies
Does not require live bacteria for accurate result
|Must be performed during first 3 weeks of illness for accurate results
Increased risk of false positivity
Allows for strain identification and antimicrobial resistance testing
|Must be performed during first 2 weeks of illness for accurate results
Requires live bacteria
||Can be performed up to 12 weeks 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