Pertussis – Diagnostics – Overview of Information and Clinical Research

Diagnosing pertussis, also known as whooping cough, requires careful attention to symptoms and proper testing methods. Early and accurate diagnosis is crucial to prevent the spread of this highly contagious respiratory illness, especially to babies and young children who are most at risk for serious complications.

Introduction: Who Should Be Tested and When

If you or your child develops a persistent cough that lasts for more than a week, especially if it comes in severe fits or is accompanied by a whooping sound when breathing in, it’s time to see a healthcare provider. Pertussis often starts like a common cold, with a runny nose, mild fever, and occasional cough, which makes it easy to miss in the early stages.^[1]

You should seek medical attention promptly if you notice symptoms that suggest whooping cough. This is particularly important if you’ve been in close contact with someone who has whooping cough, or if you belong to a high-risk group. Babies younger than one year old are at the greatest risk for severe illness and complications, so any persistent cough in an infant should be evaluated right away.^[2]

People with pre-existing health conditions that could be worsened by a respiratory infection should also be evaluated quickly. This includes those with immunocompromising conditions (conditions that weaken the immune system) and individuals with moderate to severe asthma that requires medical treatment. Pregnant women in their third trimester should also seek testing if they develop symptoms, as they could pass the infection to their newborn baby.^[1]

⚠️ Important

Don’t wait too long to seek medical care if you suspect whooping cough. Early diagnosis and treatment are most effective during the first one to two weeks of illness, before severe coughing fits begin. After three weeks, treatment may not reduce symptoms, even though it can still help prevent spreading the bacteria to others.^[11]

Even if you’ve been vaccinated against pertussis, you can still get the disease. This is because protection from the vaccine fades over time, typically weakening about 12 years after completing a vaccination series. Adolescents and adults whose immunity has waned can develop whooping cough and often spread it to babies and young children without realizing they’re infected, since their symptoms may be mild.^[5]

Classic Diagnostic Methods

When healthcare providers suspect whooping cough, they use several methods to confirm the diagnosis and distinguish it from other respiratory illnesses. The diagnostic approach depends on how long you’ve had symptoms and your overall clinical presentation.

Medical History and Physical Examination

Your healthcare provider will start by taking a detailed medical history, asking about your symptoms, when they started, and whether you’ve been exposed to anyone with whooping cough. They’ll want to know if you’ve received pertussis vaccinations and when your last dose was given. This information helps them assess your risk and determine the likelihood of pertussis.^[4]

During the physical examination, your doctor will listen to your breathing and observe your cough. The distinctive pattern of whooping cough includes rapid, repeated coughing followed by the characteristic high-pitched “whoop” sound when you try to breathe in. However, not everyone makes this whooping sound. Infants, in particular, may not whoop at all but instead may gag, struggle to breathe, or have pauses in their breathing called apnea.^[2]

Laboratory Testing: PCR Assay

The most reliable way to diagnose pertussis is through laboratory testing. A polymerase chain reaction (PCR) test has become the preferred method for confirming whooping cough. This test detects the genetic material of Bordetella pertussis, the bacteria that causes whooping cough.^[5]

To perform this test, a healthcare provider uses a special swab or a syringe filled with saline to collect a sample of mucus from the back of your throat through your nose. This is called a nasopharyngeal swab. The sample is then sent to a laboratory where technicians look for the presence of pertussis bacteria. PCR testing is more sensitive than older methods and can detect the bacteria even when they’re present in small amounts.^[4]

The timing of the test matters. PCR testing works best when done during the first three to four weeks of illness, when the bacteria are still present in your respiratory system. After this period, the bacteria may no longer be detectable even though you still have symptoms, because the ongoing cough is caused by damage the bacteria did to your airways rather than by active infection.^[5]

Culture Testing

Before PCR became widely available, bacterial culture was the standard method for diagnosing pertussis. In this test, a sample from your nose or throat is placed in a special growth medium that encourages Bordetella pertussis bacteria to multiply. After several days, laboratory technicians examine the sample to see if the bacteria have grown.^[5]

While culture testing is highly specific, meaning it rarely gives false-positive results, it has been largely replaced by PCR testing. This is because cultures take longer to produce results, typically three to seven days, and the bacteria can be difficult to grow. The bacteria are also very sensitive to how the sample is collected and transported, which can lead to false-negative results even when pertussis is present.

Blood Tests

Blood tests may be performed as part of the diagnostic workup, though they don’t directly detect the pertussis bacteria. During whooping cough, many patients develop leukocytosis (an elevated white blood cell count) with lymphocytosis (an increase in a specific type of white blood cell called lymphocytes). These changes in blood composition can support a diagnosis of pertussis, especially when combined with typical symptoms and exposure history.^[21]

Healthcare providers may also order antibody tests, which look for proteins your immune system makes in response to pertussis infection. However, these tests are less useful for diagnosing acute illness because antibodies take time to develop. They’re more helpful for confirming whether someone had pertussis in the past or for research purposes.^[5]

Chest X-Ray

In some cases, particularly when complications are suspected, your healthcare provider may order a chest X-ray. This imaging test can help identify pneumonia (lung infection), which is a common complication of whooping cough, especially in young children. A chest X-ray creates pictures of your lungs and can show areas of infection or inflammation that aren’t visible during a physical exam.^[4]

⚠️ Important

Healthcare providers may begin treatment before test results come back if your symptoms strongly suggest pertussis, if you’re at high risk for severe disease, or if you could spread the infection to vulnerable people. This is especially important for protecting infants and pregnant women in their third trimester, who face the greatest risks from whooping cough.^[11]

Distinguishing Pertussis from Other Illnesses

One of the challenges in diagnosing whooping cough is that its early symptoms look very similar to other common respiratory infections. During the first week or two, pertussis typically causes a runny or stuffy nose, red and watery eyes, mild fever, and a cough that gradually worsens. These symptoms are nearly identical to those of a common cold or other viral respiratory infections.^[2]

As the illness progresses, the cough becomes more distinctive, with rapid, uncontrollable coughing fits followed by the whooping sound or vomiting. This pattern helps healthcare providers distinguish pertussis from other conditions. The cough of whooping cough also lasts much longer than typical respiratory infections, often persisting for weeks or even months. This extended duration is why pertussis is sometimes called “the cough of 100 days.”^[5]

Other bacteria can cause similar symptoms. Bordetella parapertussis causes a milder form of whooping cough that’s difficult to distinguish from pertussis caused by Bordetella pertussis without laboratory testing. Additionally, various viruses can cause persistent coughs, though they typically don’t cause the characteristic paroxysmal coughing pattern of pertussis.^[5]

Diagnostics for Clinical Trial Qualification

While there isn’t specific information available in the sources about diagnostic criteria used specifically for qualifying patients for pertussis clinical trials, the standard diagnostic methods described above would typically form the basis for enrollment criteria. Clinical trials studying new treatments or vaccines for pertussis would likely require confirmed diagnosis through PCR testing or culture to ensure participants truly have the disease being studied.

Researchers conducting clinical trials would also document the stage of illness (catarrhal, paroxysmal, or convalescent) and the severity of symptoms to ensure they’re studying appropriate patient populations. They might use blood tests to assess immune response and measure antibody levels, particularly in trials testing new vaccines. These standardized diagnostic criteria help ensure that trial results are reliable and can be compared across different studies.

Prognosis and Survival Rate

Prognosis

The outlook for people with pertussis varies greatly depending on age and overall health. Most children older than one year and adults recover fully from whooping cough, though the cough can last for many weeks or months. The disease progresses through three distinct stages: the initial cold-like phase lasting one to two weeks, the paroxysmal stage with severe coughing fits that can last one to six weeks (sometimes up to ten weeks), and a gradual recovery period that can extend for several more weeks.^[21]

Infants younger than one year old face the most serious outcomes. More than half of babies under age one who get whooping cough require hospitalization because the infection can cause them to stop breathing. Complications in infants can include pneumonia, seizures, brain damage from lack of oxygen, and in the most severe cases, death. Babies younger than six months are at the highest risk for life-threatening complications.^[8]

Even after the acute phase of illness passes, people may experience lingering effects. Other respiratory infections can trigger the return of coughing fits for months after the initial pertussis infection, even though the bacteria are no longer present in the body. This happens because the airways were damaged during the illness and take time to fully heal. People who have had pertussis develop some immunity to future infections, though this protection isn’t complete, and it’s possible to get whooping cough more than once.^[1]

The prognosis improves significantly with early treatment. When antibiotics are started during the first one to two weeks of illness, before severe coughing fits begin, symptoms tend to be less severe. However, treatment started after three weeks of illness, while still important for preventing spread to others, typically doesn’t reduce the duration or severity of symptoms because the damage to the airways has already occurred.^[11]

Survival Rate

Deaths from whooping cough are rare in developed countries but do occur, particularly among infants. When deaths happen, they most often affect babies who haven’t yet received their full series of pertussis vaccinations. Before the pertussis vaccine was developed in the 1940s, whooping cough was a major cause of infant death. The vaccine has dramatically reduced mortality rates, but pertussis remains one of the leading causes of vaccine-preventable deaths worldwide.^[5]

Globally, there are an estimated 24 million cases of pertussis each year, resulting in more than 160,000 deaths. The vast majority of these deaths occur in low-income countries where vaccination rates are lower and access to medical care is limited. In 2018, there were more than 151,000 cases of pertussis reported worldwide.^[6]

In the United States and other countries with widespread vaccination programs, most people with pertussis survive, but the disease still poses a significant threat to the youngest and most vulnerable. The key to improving survival rates is prevention through vaccination, early diagnosis, and prompt treatment when infection occurs. Pregnant women who receive the pertussis vaccine during each pregnancy help protect their newborns during the first months of life when babies are most vulnerable.^[2]

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Pertussis – Diagnostics

Introduction: Who Should Be Tested and When