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The Test Sample
What is being tested?
Pertussis, commonly called whooping cough, is a respiratory infection caused by the bacteria Bordetella pertussis. These bacteria are highly contagious and are passed from person to person through respiratory droplets and close contact. Pertussis tests are performed to detect and diagnose a B. pertussis infection.
B. pertussis typically cause a prolonged, three-stage infection. The incubation period varies from a few days to up to three weeks.
- The first stage of the disease, called the catarrhal stage, usually lasts about two weeks and symptoms may resemble a mild cold.
- It is followed by the paroxysmal stage, which may last for one or two weeks or persist for a couple of months and is characterized by severe bouts of coughing.
- Eventually, the frequency of the coughing starts to decrease and the infected person enters the convalescent stage, with coughing decreasing over the next several weeks. Pertussis infection, however, can sometimes lead to complications such as encephalitis and seizures and it can be deadly. Infants tend to be the most severely affected and may require hospitalization.
Pertussis infections used to be very common in the United States, affecting about 200,000 people in epidemics that would occur every few years. Since the introduction of a pertussis vaccine and widespread vaccination of infants, this number has drastically decreased. (For details on the pertussis vaccine, see the Centers for Disease Control and Prevention (CDC)'s Pertussis: Summary of Vaccine Recommendations.)
However, since neither the vaccine nor the pertussis infection confers lifetime immunity, health professionals are still seeing periodic outbreaks of pertussis in young, unvaccinated infants, in adolescents, and in adults. According to the CDC, there are 10,000 to 40,000 cases reported each year. Over 48,000 cases of whooping cough were reported in 2012, the most recent peak year, and many more went unreported.
Pertussis testing is used to diagnose these infections and to help minimize their spread to others. A few different types of tests are available to detect pertussis infection:
- Detection of pertussis genetic material (polymerase chain reaction, PCR)
- Pertussis culture
- Blood tests for pertussis antibodies (serology), IgA, IgG, IgM
Pertussis can be challenging to diagnose at times because the symptoms that present during the catarrhal stage are frequently indistinguishable from those of a common cold or of another respiratory illness such as bronchitis, influenza (flu), and, in children, respiratory syncytial virus (RSV).
Many adults and vaccinated people with pertussis will present with only persistent coughing and not the classic paroxysmal cough. Suspicion of pertussis infection is increased in people who have the classic "whoop," in people who have cold symptoms and have been in close contact with someone who has been diagnosed with pertussis, and when there is a known pertussis outbreak in the community. A pertussis culture and/or PCR test will usually be ordered on these people. Testing should not be performed on close contacts who do not have symptoms.
How is the sample collected for testing?
Sample collection technique is critical in pertussis testing.
For a culture or for a test for genetic material (PCR), a nasopharyngeal (NP) swab or nasal aspirate is used. The nasopharyngeal swab is collected by having the patient tip their head back and then a Dacron swab (like a long Q-tip with a small head) is gently inserted into one of the nostrils until resistance is met. It is left in place for several seconds, then rotated several times to collect cells, and withdrawn. This is not painful, but it may tickle a bit, cause the person's eyes to tear, and provoke a coughing paroxysm.
For a nasal aspirate, a syringe is used to insert a small amount of sterile saline into the nasal passage and then gentle suction is applied to collect the resulting fluid.
For antibody testing, a blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.