How a Pertussis Test Diagnoses Whooping Cough

Diagnosing whooping cough relies on detecting the bacterium Bordetella pertussis or the body’s immune response to it, and the test your doctor chooses depends largely on how long you have been coughing. A polymerase chain reaction (PCR) test performed on a nasopharyngeal swab is the most widely used method today, but bacterial culture and blood antibody tests each fill important gaps. The reason there is no single perfect test comes down to how the infection behaves over time and how each method’s strengths line up with that timeline.

What Happens During the Swab

Nearly every pertussis test begins with a nasopharyngeal swab, and if you have never had one, it is worth knowing what to expect. A thin, flexible swab is inserted deep into the nose, all the way back to the nasopharynx, which is the space behind the nasal passages and above the throat. This is not a quick nostril swipe like some rapid flu tests. The swab needs to reach the spot where B. pertussis bacteria actually live and multiply on the ciliated epithelial cells lining that area. The swab is rotated gently and held in place for a few seconds to collect enough material.

Getting a good sample matters more than most people realize. If the swab does not reach far enough back, or if it is taken too late in the illness, the amount of bacterial material collected can be too low for reliable results. The quality of the specimen is one of the biggest variables in whether a test returns an accurate answer, regardless of how sophisticated the laboratory method is.

PCR Testing and How It Finds the Bacterium

PCR is the dominant diagnostic tool for pertussis in most developed countries. It works by amplifying tiny amounts of bacterial DNA from the swab sample until there is enough to detect. The test zeroes in on specific DNA sequences that are characteristic of B. pertussis. The most commonly targeted sequence is called IS481, a short stretch of DNA that appears in more than 50 copies within the B. pertussis genome, which makes it an extremely sensitive target.¹ Having so many copies means the test can pick up even small amounts of bacterial DNA in a sample.

Compared with older methods, PCR is far more sensitive. In one head-to-head comparison, PCR detected pertussis in about 94% of confirmed cases, while culture caught only about 15%, and direct fluorescent antibody staining landed around 52%.² Those numbers explain why PCR has become the go-to test in clinical practice. Results typically come back within hours to a day or two, fast enough to guide treatment decisions.

The Cross-Reactivity Problem

The sensitivity of targeting IS481 comes with a trade-off. B. pertussis is not the only Bordetella species that carries copies of IS481. B. holmesii, a related bacterium that can also cause respiratory illness, has eight to ten copies of IS481 in its genome, and some strains of B. bronchiseptica carry it too.¹ A test that looks only for IS481 cannot tell you for certain which species is present. In most clinical situations this does not change the treatment, since the symptoms overlap, but it matters for public health surveillance and outbreak tracking.

To get around this, many laboratories now use multiplex PCR assays that target several DNA sequences at once. One common approach combines IS481 with IS1001, which is found in B. parapertussis and B. holmesii but not in B. pertussis. By running both targets together, a lab can distinguish between B. pertussis and B. parapertussis, and partially sort out B. holmesii as well.³ Some assays go further by adding a target for the pertussis toxin gene (ptxS1), which is highly specific to B. pertussis.⁴ Others use newer targets identified through whole-genome comparisons of Bordetella species to achieve even sharper specificity.⁵

A four-target assay combining IS481, IS1001, IS1002, and recA has been developed specifically to address the limitations of single-target testing, providing a clearer picture of which Bordetella species is responsible for a given infection.⁶ The underlying principle is straightforward: the more DNA targets you check, the better you can distinguish between closely related bacterial species.

Culture, the Slow but Definitive Method

Before PCR existed, bacterial culture was the standard method for diagnosing pertussis, and it remains the only test that gives you a living bacterial isolate. This is useful for things like antibiotic susceptibility testing and genetic typing during outbreaks. The process involves inoculating the nasopharyngeal swab onto specialized agar plates, historically Bordet-Gengou medium, and incubating for several days. Modified Regan-Lowe medium is another common choice, and using both in parallel gives the best chance of growing the organism.⁷

The big drawback of culture is that it misses most cases. With a sensitivity of only about 15% in one comparative study, culture fails to detect the vast majority of pertussis infections.² B. pertussis is a finicky organism that dies quickly once outside the body, so delays in transporting specimens to the lab, prior antibiotic use, or poor swab technique can all kill the bacteria before they reach the culture plate. And even under ideal conditions, you are waiting three to seven days for colonies to appear. In practice, culture is now used primarily as a backup alongside PCR, not as the frontline test.

Blood Tests for Later in the Illness

PCR and culture both work best during the first two to three weeks of illness, when live bacteria and bacterial DNA are most abundant in the nasopharynx. But many people do not seek testing until they have been coughing for weeks, especially adults who may not recognize their symptoms as pertussis. By that point, the bacteria have often been cleared or greatly reduced, and swab-based tests become unreliable. This is where serological testing, which measures antibodies in the blood, fills the gap.

The most widely recommended serological approach measures IgG antibodies against pertussis toxin (PT), a protein produced only by B. pertussis. European reference laboratories recommend using a diagnostic cutoff for IgG-anti-PT between 50 and 120 international units per milliliter on a single blood sample.⁸ A level above the cutoff in someone with a compatible cough illness is considered diagnostic. If the IgG result is ambiguous, IgA antibodies against PT can be checked as a secondary marker.⁸

In a recent study of adult whooping cough cases, ELISA-based antibody testing confirmed the diagnosis in over 96% of patients, compared with only about 22% by PCR.⁹ That enormous gap reflects the fact that most adults are tested late in the course of illness, when the bacteria are largely gone but the immune response is peaking. The ELISA method showed significantly higher diagnostic value starting from the third week of illness onward.⁹

One important limitation: serological testing cannot be reliably interpreted within a year of receiving an acellular pertussis vaccine, because the vaccine itself triggers production of the same IgG-anti-PT antibodies the test measures.⁸ If you received a Tdap booster within the past year and then develop a persistent cough, your doctor would need to rely on PCR or culture rather than a blood antibody test to confirm pertussis.

Why Your Age Affects Test Results

The amount of bacteria in the nasopharynx varies dramatically by age, and this has real consequences for how well swab-based tests perform. In one quantitative study, the average number of bacterial cells recovered from a nasopharyngeal swab in infants was estimated at roughly one million, compared with about 21,000 in older children and only about 320 in adults.¹⁰ That is a difference of several orders of magnitude.

This matters for testing in two ways. First, in adults with very low bacterial loads, PCR tests may return weak positive results or false negatives, especially if the swab was not perfectly collected. Second, the dramatically lower bacterial burden in adults helps explain why adult pertussis tends to present with milder, less classic symptoms, which in turn makes clinicians less likely to order the test in the first place. The disease often masquerades as a stubborn cough in adults, and by the time someone thinks to test for pertussis, they may be past the window where PCR works well.

How Antibiotics Complicate the Picture

If your doctor starts you on antibiotics before a pertussis test is collected, the treatment begins killing bacteria and reducing the amount of DNA available for PCR to detect. But bacterial DNA does not disappear immediately. In a study of young children treated with antibiotics, PCR remained positive in all patients after five days of treatment. Even after 14 days, about 83% still tested positive, and after 21 days, two thirds still had detectable B. pertussis DNA.¹¹ In one case, bacterial DNA was still detectable a full month after treatment began.

The practical takeaway is that starting antibiotics before the swab is collected does not necessarily make the test useless, though it does reduce the amount of DNA present and could tip a borderline result to negative. The DNA concentration dropped steadily during treatment, so the sooner the swab is collected relative to starting antibiotics, the better the chances of getting a clear result. Culture, on the other hand, is much more vulnerable to prior antibiotic use, because it requires living bacteria, which are killed quickly by appropriate antibiotics.

Syndromic Respiratory Panels

You may encounter pertussis testing in a setting you did not expect: multiplex respiratory panels. These are tests designed to screen for many respiratory pathogens at once using a single nasopharyngeal swab. The BioFire FilmArray Respiratory Panel 2plus, for example, tests for 22 pathogens simultaneously, including 18 viruses and 4 bacteria, with B. pertussis among them.¹² The system automates DNA extraction, amplification, and analysis within a single closed cartridge, and returns results in about an hour.¹³

These panels are convenient and increasingly common in emergency departments, but there is a catch when it comes to pertussis specifically. The detection rate for B. pertussis on syndromic panels has been reported as low compared with dedicated pertussis PCR assays.¹² If your doctor suspects whooping cough based on your symptoms and clinical history, a dedicated pertussis PCR is a better bet than relying on a broad panel. The multiplex panel is designed to cast a wide net for common respiratory infections, not to optimize sensitivity for any one organism.

A Surprising Source of False Positives

One of the more unusual pitfalls of pertussis PCR testing involves vaccine contamination. Pertussis vaccines, both childhood combination vaccines like Pentacel and adult boosters like Adacel, contain high copy numbers of B. pertussis DNA. If these vaccines are handled in the same area where PCR samples are processed, aerosolized vaccine material can contaminate specimens and produce false-positive results.¹⁴

This is not a theoretical concern. It was identified through investigation after unexpectedly high rates of positive PCR results in a setting where vaccines were being administered nearby. The DNA in the vaccine is not from live bacteria and poses no infection risk, but it is perfectly capable of triggering a PCR assay that is designed to detect even tiny amounts of pertussis DNA. Laboratories and clinics that perform pertussis testing need to keep vaccine handling physically separated from specimen collection and processing areas.

Clinical Diagnosis Without Laboratory Confirmation

In many parts of the world, laboratory confirmation of pertussis is not available or is not obtained before treatment decisions need to be made. Clinical case definitions exist for this reason. The Global Pertussis Initiative developed an algorithm that defines the signs and symptoms most suggestive of pertussis for three age groups: zero to three months, four months to nine years, and ten years and older.¹⁵ These definitions recognize that the classic “whoop” is common in young children but frequently absent in older children and adults.

Clinical case definitions are useful for surveillance and for guiding empiric treatment, but their accuracy as standalone diagnostic tools is limited. When tested against laboratory-confirmed cases in infants and young children with severe respiratory infections, the WHO clinical criteria had a sensitivity of about 78% but a specificity of only about 16%, meaning they captured most true cases but also flagged many children who did not actually have pertussis. The Global Pertussis Initiative criteria performed differently, with a sensitivity of about 34% and a specificity of about 65%.¹⁶ Neither set of clinical criteria alone approached the reliability of laboratory testing. Where laboratory testing is available, it remains essential for confident diagnosis.

Emerging Point-of-Care Technologies

Standard pertussis PCR requires a laboratory with thermocycling equipment and trained technicians, which limits how quickly results can reach the bedside and makes the test impractical in remote or low-resource settings. Researchers have been working on alternatives that could bring molecular diagnosis closer to the patient.

One approach uses loop-mediated isothermal amplification, a technique that amplifies DNA at a constant temperature rather than requiring the rapid heating and cooling cycles of traditional PCR. A validated LAMP assay for pertussis was shown to be about two and a half times faster than real-time PCR while maintaining comparable sensitivity and specificity.¹⁷ Researchers have taken this further by integrating LAMP into a low-cost, 3D-printed microfluidic device powered by a portable battery-operated heater. The device produces fluorescent results visible to the naked eye in about an hour, without any external instruments or electricity.¹⁸

These technologies are still in development and validation stages, not yet part of routine clinical practice. But they represent a path toward diagnosing whooping cough in settings where sending a swab to a reference laboratory and waiting days for results is not feasible. For pertussis, where early diagnosis matters both for treatment and for protecting vulnerable contacts like young infants, faster turnaround could make a real difference in outcomes.

Matching the Test to the Moment

The practical reality of pertussis diagnosis is that no single test covers all situations well. Here is a rough guide to what works when:

First one to three weeks of cough: PCR on a nasopharyngeal swab is the best option. Culture can be run in parallel if a bacterial isolate is needed, but expect PCR to be far more sensitive.
After three weeks of cough: Serology becomes the primary tool, especially in adolescents and adults. PCR sensitivity drops as the bacterial load in the nasopharynx declines.
Infants with acute illness: Both PCR and culture tend to perform well because bacterial loads are high. Culture is particularly valuable in this group for monitoring antibiotic resistance patterns.
Recent vaccine recipients: Blood antibody tests are unreliable for at least a year after acellular pertussis vaccination. PCR is the better choice.

Physicians sometimes order both a PCR and a serological test simultaneously when the timing of illness onset is uncertain. The two methods complement each other because they are measuring fundamentally different things: one looks for the bacterium’s DNA, and the other looks for the immune system’s response to the bacterium. A negative PCR in someone with a four-week cough does not rule out pertussis if the serology comes back elevated. Understanding this interplay is especially relevant for adults, who tend to present later and carry lower bacterial burdens, making them the group most likely to fall through the diagnostic cracks if only one testing method is used.