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 coughing and sneezing and close contact. Whooping cough tests are performed to detect and diagnose infection with B. pertussis.
The time between infection with the bacteria and developing signs and symptoms (incubation period) varies from a few days to up to three weeks. B. pertussis typically causes a prolonged, three-stage infection:
- Catarrhal Stage: This is the first stage of the disease, which usually lasts about one to two weeks. Symptoms may resemble a mild cold, with runny nose, low-grade fever and an occasional cough. Infants may develop apnea (a pause in breathing) during this stage and may be more prone to choking. Individuals are highly contagious during this time.
- Paroxysmal Stage: This is the second stage of infection, which may last for one to two weeks or persist for a couple of months, and is characterized by severe bouts of coughing. Coughing is frequently followed by a ‘whoop’ sound and patients may feel exhausted and/or vomit after these coughing fits.
- Convalescent Stage: During the third stage, the frequency and severity of the coughing starts to lessen and decreases over the next two to three weeks.
Whooping cough can sometimes lead to complications requiring hospitalization, particularly for infants, and may include apnea (61%) or pneumonia (23%), or much less frequently convulsions (1%), brain disease or damage (encephalopathy) (about 0.3%) or death (1%).
Whooping cough used to be very common in the United States, affecting about 200,000 people during outbreaks that would occur every few years. Since the introduction of a whooping cough vaccine and widespread vaccination of infants, this number has drastically decreased. For details on the whooping cough vaccine, see the Centers for Disease Control and Prevention (CDC)’s Pertussis: Summary of Vaccine Recommendations.
Importantly, prior infection or receipt of the complete vaccination series for whooping cough do not prevent you from becoming infected as they do not provide lifetime immunity or protection from re-infection. Fully vaccinated individuals who are infected with B. pertussis may develop a less severe infection, with persistent coughing but without the classic paroxysmal cough.
Periodic outbreaks of whooping cough in unvaccinated infants, in adolescents, and in adults still occur in the United States. According to the CDC, 10,000 to 40,000 cases of whooping cough are reported each year. Over 48,000 cases of whooping cough were reported in 2012, the most recent peak year, and many more likely went unreported.
Diagnostic tests for whooping cough
Because the initial symptoms of whooping cough, especially during the catarrhal stage, are frequently indistinguishable from those of a common cold or of other respiratory illnesses, such as bronchitis, influenza (flu), or in children, respiratory syncytial virus (RSV), diagnostic testing is necessary to help identify the infection.
There are a few test methods that may be used to detect whooping cough, and the type of testing your healthcare practitioner orders is largely dependent on how long you have had symptoms.
- Bacterial culture – if the B. pertussis bacteria are present in your sample, this test will grow them on an agar plate. It may take up to 7 days for the bacteria to grow; however, the bacteria may be less likely to grow if you have been treated with certain antibiotics before the sample was collected. Culture testing is most useful in the first 2 weeks after symptom onset. If necessary, bacteria that grow in culture may be used for susceptibility testing to identify the optimal antibiotic to use.
- Polymerase chain reaction (PCR) – this test detects genetic material (e.g., DNA) from the bacteria. The test can be completed within a couple of hours. PCR can detect genetic material from the bacteria from the time of initial symptom onset to approximately 3-4 weeks later.
- Blood test for antibodies (i.e., IgA, IgG, IgM) to pertussis (serology) – these tests detect antibodies produced by the body’s immune system in response to B. pertussis. Antibody testing is not the preferred test because it is an indirect method to identify the infection. This type of testing is most useful in patients with more than 3-4 weeks of symptoms. According to the CDC, serology can be used from 2 to 8 weeks after the cough starts, and in some cases may be helpful up to 12 weeks after onset.
How is the sample collected for testing?
Proper collection of the appropriate samples is essential for accurate whooping cough test results.
For bacterial culture or testing by PCR, a nasopharyngeal (NP) swab or nasal aspirate is collected. For tests to detect antibodies to the bacteria, a blood sample is needed.
- The nasopharyngeal swab is collected by having you tip your head back and then a swab (like a long Q-tip with a small head) is gently inserted through one of your 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, cause your eyes to tear, and provoke a coughing spell.
- 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.
How are they used?
Whooping cough tests are used to detect and diagnose a B. pertussis infection. Importantly, early diagnosis and treatment may lessen the severity of symptoms and help limit spread of the disease.
When are they ordered?
Whooping cough tests may be ordered when you have signs and symptoms that suggest a pertussis infection. Your health practitioner may have a strong suspicion that you have whooping cough if:
- You have the classic “whoop,”
- You have cold symptoms and have been in close contact with someone who has been diagnosed with whooping cough
- There is a known whooping cough outbreak in the community
Testing should not be performed on close contacts who do not have symptoms.
What does the test result mean?
- A positive PCR test means that genetic material (i.e., DNA) from B. pertussis was detected in your specimen, indicating that you have been infected. However, the PCR test may also be positive with other Bordetella species.
- A negative PCR test means that it is less likely that you have whooping cough but does not rule it out. If there are very few bacteria present in the sample, they may not be detected by PCR.
- A positive culture is diagnostic for whooping cough.
- Similar to PCR, a negative culture does not rule out whooping cough. Culture results are dependent on proper specimen collection and transport, how long you have had symptoms prior to collection, and whether there was any prior antibiotic treatment before the sample was collected.
Both culture and PCR tests are less likely to be positive as the illness progresses.
With blood testing, pertussis IgG antibodies will be present in those who have been vaccinated or have had a past infection. IgM and IgA antibodies may indicate recent vaccination or infection and will only be present for a short period of time (2-3 months). Blood testing is not typically recommended for the diagnosis of active whooping cough and is most useful after approximately 3-4 weeks of symptoms.
How is a whooping cough treated?
Whooping cough is treated with antibiotics, which will help to resolve the infection and help stop spread of the disease. Close contacts of people diagnosed with whooping cough may also be treated to prevent spread of the infection.
When is the pertussis vaccine given?
Three different formulations of the pertussis vaccine are available, typically in combination with the tetanus and diphtheria vaccines (DPT). The vaccination is given to infants as a series of shots. Children who have not completed the series of vaccinations are at a higher risk of becoming infected. Even some people who have been vaccinated may still be infected by B. pertussis, but they tend to have a less severe illness.
Pregnant women are advised to be re-vaccinated to prevent transmission of pertussis to their newborns. Grandparents and other caretakers who will be spending time with a newborn are also advised to be re-vaccinated.
Why is my travel history important to my healthcare practitioner?
International travelers should be aware that many resource-limited countries do not have widespread vaccination programs for whooping cough. Infants who have not completed their series of vaccinations and people who have not had a recent booster vaccine may be at an increased risk of contracting whooping cough.
Can a throat swab be used instead of a nasopharyngeal sample from my nose?
A throat swab is not acceptable. During a pertussis infection, the bacteria are found in the back of the nose, not in the throat or the front portion of the nose.
Can whooping cough testing be done in my healthcare practitioner's office?
No. There is no simple, rapid diagnostic test for whooping cough. It requires specialized equipment and is typically performed in clinical laboratories. Not every laboratory performs this testing and samples may need to be sent to a public health laboratory.
Why did my doctor report my child's whooping cough?
Health practitioners are required to report whooping cough to state health departments. Outbreaks are tracked and interventions, such as vaccination and appropriate prophylactic antibiotic treatment, may be provided to help limit the outbreak.
My doctor said I have Bordetella parapertussis. Is this the same as whooping cough?
B. parapertussis are bacteria that can infect humans in the same manner as B. pertussis, but the infection usually causes a milder respiratory illness. Culture methods and PCR tests can detect and distinguish B. parapertussis from B. pertussis, and both are commonly tested for since the signs and symptoms may be similar in people with either infection. Blood tests may not detect antibodies to B. parapertussis and there is no vaccine to prevent B. parapertussis infection.
Is there anything else I should know?
Direct fluorescent antibody (DFA) is a testing method that is no longer recommended or routinely available in clinical laboratories to detect whooping cough. This method is less specific and less sensitive than either the culture or PCR methods.