Introduction
Pertussis, also called whooping cough, is usually identified through a combination of clinical suspicion and laboratory confirmation. The disease is caused by Bordetella pertussis, a bacterium that attaches to the lining of the respiratory tract and releases toxins that disrupt normal airway function. Those toxins help explain why the illness can produce prolonged coughing episodes, airway irritation, and a distinctive pattern of symptoms that often changes over time.
Accurate diagnosis matters for two main reasons. First, pertussis is highly contagious, especially early in the illness before the diagnosis is obvious. Identifying it promptly allows doctors to reduce spread to close contacts, infants, pregnant people, and others at higher risk of severe disease. Second, treatment is most useful when given early, and confirmation helps distinguish pertussis from many other causes of cough that are managed differently.
Recognizing Possible Signs of the Condition
Doctors usually begin to suspect pertussis when the pattern of illness fits the known stages of infection. In the early stage, symptoms may resemble an ordinary upper respiratory infection: runny nose, mild fever, sneezing, and a cough that is not yet severe. This stage can be misleading because the illness may not yet have the classic features that make pertussis recognizable.
As the disease progresses, the cough often becomes more intense and repetitive. A person may have coughing fits, also called paroxysms, that occur in rapid bursts with little ability to catch a breath between coughs. Some patients develop a high-pitched inspiratory sound after a coughing spell, traditionally described as a “whoop,” although this is less common in vaccinated children, adolescents, and adults. Vomiting after coughing, exhaustion after an episode, and a cough that persists for weeks are additional clues.
In infants, pertussis may not follow the classic pattern. They may have brief pauses in breathing, feeding difficulty, cyanosis, or only mild coughing rather than obvious “whooping.” Because infants can deteriorate quickly, clinicians keep a lower threshold for testing when the exposure history or symptom pattern suggests pertussis. Adults and older children may present with prolonged cough without dramatic paroxysms, which can make the diagnosis easier to miss unless there is a known contact or an outbreak.
Medical History and Physical Examination
The diagnostic process starts with a careful history. Healthcare professionals ask how long the cough has been present, whether it comes in fits, and whether there has been vomiting after coughing, breathing difficulty, or a known exposure to someone with confirmed pertussis. They also ask about vaccination status, because immunization lowers the risk of severe disease but does not fully eliminate infection. A history of incomplete vaccination, a waning immune response over time, or contact with vulnerable infants may increase suspicion.
Clinicians also look for the timing of symptoms. Pertussis is often most recognizable during the paroxysmal phase, which may begin after a week or two of nonspecific cold-like symptoms. A cough lasting more than two weeks, especially if it worsens at night or is associated with posttussive vomiting, raises concern. Recent outbreaks in schools, households, or healthcare settings can be important because pertussis spreads efficiently in close-contact environments.
During the physical examination, the doctor evaluates breathing rate, oxygen level, and signs of distress. The exam may be relatively normal between coughing spells, which is one reason the condition can be difficult to identify on appearance alone. In infants, clinicians watch closely for apnea, poor feeding, or fatigue. In more severe illness, repeated coughing can cause conjunctival bleeding, facial swelling, or small petechial hemorrhages from the pressure generated during the cough. These findings are not specific to pertussis, but they support the diagnosis when combined with the history.
Diagnostic Tests Used for Pertussis
Laboratory testing is the main way pertussis is confirmed. The most common test is a nasopharyngeal swab or aspirate collected from the back of the nose, where Bordetella pertussis is most likely to be present early in the illness. The choice of specimen matters because a throat swab is less reliable. The sample can be used for polymerase chain reaction, culture, or sometimes both.
Polymerase chain reaction (PCR) is widely used because it detects genetic material from the bacteria quickly and with good sensitivity, especially in the first few weeks of cough. PCR can provide results faster than culture, which helps clinicians decide on treatment and public health measures. Its limitation is that it may detect nonviable bacterial DNA, so a positive result must be interpreted in the clinical context. PCR is also most useful before or soon after antibiotic treatment begins and after the cough has not been present for too long.
Culture remains the most specific test and can provide definitive laboratory evidence of infection. The organism is grown from the nasopharyngeal sample on special media. Culture is particularly valuable for confirming cases during outbreaks and for public health surveillance because it identifies the organism directly. However, it is slower and less sensitive than PCR, especially if the patient has been ill for several weeks, has already taken antibiotics, or if sample collection was delayed. Because Bordetella pertussis is fastidious, culture requires careful technique and prompt transport.
Serologic testing may be used later in the course of illness, especially when PCR and culture are less likely to be positive. Serology measures antibodies against pertussis components, most commonly pertussis toxin. A rise in antibody levels, or a high level consistent with recent infection, can support the diagnosis. This test is less useful in very young infants and can be harder to interpret in people who were recently vaccinated, because vaccination may affect antibody levels. For that reason, serology is usually an adjunct rather than the primary test.
Complete blood count can provide supportive information. Pertussis sometimes causes marked lymphocytosis, especially in infants. This reflects the effect of pertussis toxin, which alters immune cell movement and contributes to the characteristic blood abnormality. A high white blood cell count does not confirm the disease, but in a sick infant with cough or apnea, it can strengthen concern and help assess severity.
Imaging tests are not used to diagnose pertussis itself, but chest radiography may be ordered when complications or alternative diagnoses are being considered. A chest X-ray can show peribronchial thickening, atelectasis, or signs of pneumonia if there is a secondary bacterial infection. Many patients with uncomplicated pertussis have a normal image. Thus, imaging is mainly helpful when the clinician needs to evaluate distress, hypoxia, or suspected pneumonia rather than to prove pertussis directly.
Functional tests are not central to diagnosis, but pulse oximetry is often used to evaluate oxygenation, especially in infants or patients with severe coughing spells. In unusual cases, pulmonary function testing may be considered in older patients if the diagnosis is uncertain, but it is not a standard confirmatory test. Because the illness is infectious and often diagnosed clinically plus by laboratory testing, functional studies rarely play a primary role.
Tissue examination is not routinely used. Pertussis is diagnosed from respiratory samples and clinical findings rather than biopsy. Microscopic examination of tissue does not usually add value in routine cases.
Interpreting Diagnostic Results
Doctors interpret pertussis testing by combining the result with the stage of illness and the likelihood of exposure. A positive PCR or culture from an appropriate nasopharyngeal specimen in a patient with compatible symptoms is strong evidence of pertussis. Culture is especially persuasive because it isolates the organism, but a negative culture does not exclude disease, particularly if testing was delayed. PCR positivity is generally helpful earlier in illness, but an isolated positive result in someone with few symptoms may require clinical correlation.
Negative test results do not always rule out pertussis. If the sample was poorly collected, taken too late, or obtained after antibiotics were started, the bacteria may no longer be detectable. Serologic results must also be interpreted carefully. A positive antibody level may indicate recent infection, but depending on the assay and vaccination history, it may be difficult to know whether antibodies reflect infection, immunization, or both. Clinicians therefore use test timing, symptom pattern, and exposure history to judge whether the result supports an active case.
In infants, a high white blood cell count with lymphocytosis and respiratory symptoms may increase concern even before microbiologic confirmation returns. In adults, the diagnosis may be considered probable if the cough is prolonged and the laboratory tests fit the timing of illness, even when no classic whoop is present.
Conditions That May Need to Be Distinguished
Several disorders can cause a prolonged or severe cough and may resemble pertussis. Viral upper respiratory infections are common early on, but they usually improve sooner and do not typically produce the same pattern of coughing fits and posttussive vomiting. Acute bronchitis can also cause persistent cough, but it is usually associated with airway inflammation rather than the toxin-mediated cough paroxysms seen in pertussis.
Pneumonia may be considered when fever, abnormal lung findings, low oxygen saturation, or infiltrates on chest imaging are present. Asthma can cause coughing spells, wheezing, and nighttime symptoms, but it usually has a history of variable airflow limitation and response to bronchodilators. Gastroesophageal reflux and postnasal drip can produce chronic cough, yet they do not cause the same contagious outbreak pattern or the same respiratory sample results.
In infants, bronchiolitis, respiratory syncytial virus infection, and other viral illnesses may look similar because they can cause apnea, poor feeding, and cough. Tuberculosis is another important consideration in prolonged cough, especially if exposure risk exists, though the clinical course and testing strategy differ. Clinicians separate these conditions by integrating age, vaccination history, exposure, symptom timing, exam findings, and targeted laboratory or imaging results.
Factors That Influence Diagnosis
Several factors change how pertussis is diagnosed and how easy it is to confirm. Timing is one of the most important. PCR and culture are most useful earlier, while serology becomes more useful later. If testing is delayed, the chance of a definitive result declines. Prior antibiotic therapy can also reduce the yield of culture and PCR.
Age matters as well. Infants may show apnea or minimal cough rather than classic paroxysms, so clinicians often test based on a lower threshold of suspicion. Adolescents and adults may have milder or atypical symptoms, especially if they were vaccinated in the past. In these groups, the diagnosis often depends on asking specifically about exposure and the duration of cough.
Vaccination history affects both presentation and interpretation. Vaccinated people can still get pertussis, but symptoms may be less typical. Vaccination can also influence serologic testing, so the clinician must know whether the patient recently received a pertussis-containing vaccine. Underlying medical conditions such as chronic lung disease, immune compromise, or pregnancy may prompt more urgent testing and closer follow-up because the consequences of missed diagnosis are greater.
Conclusion
Pertussis is diagnosed by combining clinical reasoning with targeted testing. Doctors first look for the characteristic pattern of prolonged cough, paroxysms, posttussive vomiting, exposure history, and age-specific signs such as apnea in infants. They then confirm the diagnosis with laboratory tests, usually PCR or culture from a nasopharyngeal specimen, and sometimes serology later in the illness. Imaging and oxygen assessment may help evaluate severity or complications, but they do not confirm the infection on their own.
Because the illness changes over time and can appear differently in vaccinated people, accurate diagnosis depends on matching the test to the stage of disease and interpreting results in context. When history, examination, and laboratory evidence are considered together, medical professionals can identify pertussis more reliably and distinguish it from other causes of cough.