Dr Leigh Martens

General Practitioner

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Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Pertussis


At a Glance

Why Get Tested?

To detect and diagnose a Bordetella pertussis infection, commonly known as whooping cough

When to Get Tested?

When you have persistent spasms or fits of coughing (paroxysms) that the healthcare practitioner suspects is due to pertussis (whooping cough); when you have symptoms of a cold and have been exposed to someone with pertussis

Sample Required?

A nasopharyngeal (NP) swab or a nasal aspirate; occasionally, a blood sample drawn from a vein in your arm

Test Preparation Needed?

None

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.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

Pertussis tests are used to detect and diagnose a Bordetella pertussis infection, commonly known as whooping cough, a highly contagious infection of the respiratory tract. Early diagnosis and treatment may lessen the severity of symptoms and help limit spread of the disease. (For more on the infection, see the "What is being tested?" section.)

There are a few test methods that may be used to detect a pertussis infection:

  • Polymerase chain reaction (PCR) – this test amplifies the genetic material of the bacteria when present in a sample so that it can be detected. Results are available within a couple of days. False-positive results may occur when PCR is used. Therefore, the Centers for Disease Control and Prevention (CDC) recommend that during a suspected outbreak, at least one case be confirmed using culture.
  • Culture – this test was the "gold standard" for identifying pertussis before the advent of the PCR assay and it may be used to diagnose pertussis early in the infection. The sample is put into nutrient media and the bacteria are grown and identified. Results are reported in one to two weeks. A pertussis culture will allow the recovery of the bacteria in order to investigate potential outbreaks. Susceptibility testing can also be performed to determine which antibiotic to use for treatment. Typically, a pertussis culture and PCR test will both be ordered, as early in the illness as possible. Cultures are less likely to grow the bacteria two to three weeks into the illness and will be affected by some antibiotics if the person has been treated.
  • Blood test for pertussis antibodies, IgA, IgG, IgM – these blood tests detect antibodies produced by the body's immune response to a pertussis infection. Antibody testing is not useful to determine if a person has had a recent pertussis infection. Pertussis IgG antibodies will be present in anyone who has been vaccinated. Pertussis IgM and IgA antibodies will usually only be present a short time after vaccination or infection. These tests may be performed in a research setting to evaluate and study the spread of pertussis in the community. Rarely, an antibody test may be performed to evaluate the adequacy of a person's immune response to a pertussis vaccine.

When is it ordered?

Pertussis tests are ordered when someone has signs and symptoms suggestive of pertussis, and as early in the illness as possible (generally within the first two weeks for culture and the first three weeks for PCR).

Symptoms during the first stage of the infection, called the catarrhal stage, may include typical cold symptoms such as a runny nose, sneezing, mild cough, and/or a low-grade fever. After about two weeks, the paroxysmal stage begins and may include symptoms such as:

  • Frequent severe bouts of coughing, sometimes followed by vomiting
  • Several rapid coughs followed by a whooping sound as the person inhales; affected adults may cough but not whoop, and infants may have trouble breathing and may choke more than whoop.

These symptoms may last for one or two weeks or persist for a couple of months. During the convalescent stage, the severity of symptoms lessens, with the frequency of coughing gradually decreasing over the next several weeks.

What does the test result mean?

A positive PCR test means that it is likely that the person has pertussis. However, the PCR test may also be positive with other Bordetella species. A negative PCR test means that it is less likely that the person has pertussis but does not rule it out. If there are an insufficient number of bacteria in the sample, then they may not be detected.

A positive culture is diagnostic for a B. pertussis infection, but a negative culture does not rule it out. Culture results are dependent on proper specimen collection and transport, duration of symptoms, when the sample is collected, and prior antimicrobial therapy administered before the culture is taken.

Both culture and PCR tests are less likely to be positive as the illness progresses.

Is there anything else I should know?

The pertussis vaccination is given to infants as a series of shots. Those children who have not completed the series of pertussis vaccinations are at a higher risk of becoming infected. Even some people who have been vaccinated may be infected by Bordetella pertussis, but they will tend to have a less severe illness.

Pertussis is treated with antibiotics, which will help to resolve the infection and help stop spread of the disease.

International travelers should be aware that many less developed countries do not have widespread vaccination for pertussis. Infants who have not completed their series of vaccinations and people who have not had a booster vaccination in many years may be at an increased risk of contracting pertussis.

Pregnant women are advised to be re-vaccinated to prevent transmission of pertussis to the newborn. Grandparents and other caretakers who will be spending time with a newborn are also advised to be re-vaccinated.

Direct fluorescent antibody (DFA) is a test method that is no longer recommended to detect pertussis infections since it is less specific and sensitive than either the pertussis culture or PCR.