Pertussis (whooping cough) is a respiratory infection caused by the bacterium Bordetella pertussis. It spreads through airborne droplets from coughing or sneezing. While it can affect people of all ages, it is particularly dangerous for infants. Because initial symptoms resemble a common cold, laboratory testing is the only way to confirm a diagnosis. This confirmation allows for appropriate treatment and helps prevent further transmission within the community.
Recognizing Pertussis Symptoms
The signs of pertussis evolve through distinct stages. The first phase, the catarrhal stage, typically lasts one to two weeks and presents with mild, cold-like symptoms. These include a runny nose, low-grade fever, and an occasional cough.
Following the initial phase, the illness progresses into the paroxysmal stage, which can last from one to six weeks. This stage is defined by fits of numerous, rapid coughs. After a coughing spell, a person may struggle to breathe in, causing a high-pitched “whoop” sound. These severe coughing fits, which often worsen at night, can lead to exhaustion, vomiting, and in infants, a bluish skin tone from lack of oxygen.
The final stage is the convalescent, or recovery, stage. This stage can last for weeks to months, and the cough gradually lessens in severity and frequency. However, for many months after the initial illness, subsequent respiratory infections can trigger a return of the paroxysmal coughs. Symptoms vary; vaccinated individuals may have a milder cough without the “whoop,” and infants may not whoop but instead experience apnea, which are dangerous pauses in breathing.
Types of Diagnostic Tests
Laboratories use several methods to confirm a pertussis diagnosis, each with applications depending on the timing of the illness. The most widely used and preferred method is the Polymerase Chain Reaction (PCR) test. This sensitive technique detects the genetic material (DNA) of the Bordetella pertussis bacteria from a respiratory sample. PCR tests can provide results quickly and are most effective within the first three to four weeks of the cough’s onset.
Another diagnostic tool is a bacterial culture, which was once considered the gold standard for diagnosis. This method involves growing Bordetella pertussis from a patient’s sample in a lab. While a positive culture is 100% specific for pertussis, it is less sensitive than PCR and the process is slower. The likelihood of successfully culturing the bacteria decreases significantly after the first two weeks of illness.
Serology is a blood test that looks for antibodies the body produces to fight the infection, not the bacteria itself. Specifically, it measures IgG antibodies to the pertussis toxin. This test is most useful later in the illness, typically two to eight weeks after the cough begins, when antibody levels are highest. It is not ideal for diagnosing an active infection in its early stages but can help confirm a diagnosis retrospectively.
The Testing Procedure
The sample for the most common pertussis tests is collected from the nasopharynx, the upper part of the throat behind the nose where the bacteria are most likely to be found. The standard collection method uses a nasopharyngeal (NP) swab.
To collect the sample, a healthcare provider will ask the patient to tilt their head back slightly. The swab is gently inserted into one nostril and advanced straight back along the floor of the nasal passage until it reaches the nasopharynx. The swab is left in place for a few seconds and gently rotated to absorb secretions before being slowly removed.
While the procedure can be uncomfortable, it is very brief. An alternative method, sometimes used for young children, is a nasal aspirate. This involves flushing a small amount of saline solution into the nasal cavity and then collecting the fluid. Both the NP swab and aspirate can provide a sample suitable for either PCR testing or a bacterial culture.
Understanding Your Test Results
A positive PCR or culture test confirms an active whooping cough infection, and a healthcare provider will prescribe a course of antibiotics to treat the illness and reduce the period of communicability.
A negative result indicates that the bacteria were not detected in the sample. However, a negative test does not completely rule out pertussis, especially if the sample was collected late in the course of the illness. A provider will consider the test result alongside clinical symptoms and known exposure to make a final diagnosis. For serology, a positive result suggests a recent infection, but vaccination history must be considered for accurate interpretation.
The time it takes to receive results varies by the type of test performed. PCR results are generally available within one to two days, allowing for a rapid diagnosis. Culture results take longer, as the bacteria need time to grow, with final results often taking up to a week.