What a High B. Pertussis IgG Antibody Result Means

A high IgG antibody result against Bordetella pertussis almost always points to a recent encounter with the bacterium that causes whooping cough. In clinical practice, the test that matters most measures IgG directed specifically at pertussis toxin (PT), and a single-sample level at or above roughly 100 IU/mL is widely considered diagnostic of recent or active B. pertussis infection.¹ But interpreting the number is not as simple as reading it off the lab report. How high the level climbs, how long it stays elevated, and whether it was driven by infection or vaccination all shape what the result actually tells you.

What the Test Measures

The standard approach to serological diagnosis of pertussis uses an ELISA test that detects IgG antibodies against pertussis toxin in a blood sample.² Pertussis toxin is the key target because it is produced only by B. pertussis, making antibodies against it the most specific marker of pertussis infection available through blood testing.³ Other B. pertussis proteins like filamentous hemagglutinin (FHA) and pertactin also trigger an immune response, but antibodies to those proteins can cross-react with infections from related bacteria or simply reflect vaccination, so they are less useful for diagnosing a current infection.

Some labs also measure IgA or IgM antibodies to pertussis antigens. IgA against PT can support a diagnosis when combined with IgG, and IgM tends to appear early but is not consistently detected in adults. Testing all three antibody classes together with multiple antigens has shown the highest overall sensitivity, though in practice most clinical labs focus on IgG anti-PT because it strikes a good balance between specificity and availability.⁴

What Counts as “High”

Different labs and different countries use slightly different numbers, which is one of the more frustrating aspects of pertussis serology. Two thresholds show up most often in the research literature and in clinical guidance. A level at or above about 100 IU/mL (sometimes reported as EU/mL) on a single blood draw is considered diagnostic of recent or active infection, with that threshold present in fewer than one percent of the general population.¹ A large U.S. study proposed a cutoff of 94 EU/mL, which yielded about 80 percent sensitivity and 93 percent specificity when tested against culture-confirmed pertussis cases.⁵

Some European reference labs have explored a lower optimum cutoff around 68 IU/mL, which captures more true infections but also flags a few more false positives. Using a higher cutoff of 125 IU/mL tightened specificity to nearly 99 percent while still catching about 88 percent of infections in a large analysis of over 14,000 serum samples.⁶ In routine clinical use, most guidelines settle on a threshold in the range of 100 to 125 IU/mL as a practical compromise between sensitivity and specificity.⁷

Results that fall below the diagnostic cutoff but are still elevated compared to the general population sit in a gray zone. A level of, say, 50 IU/mL might reflect a past infection that is no longer recent, an early infection where antibodies have not yet peaked, or a strong response to vaccination. Without a second sample drawn weeks later to look for a rise or fall, the single number alone cannot resolve the ambiguity.

Timing Is Everything

One of the biggest pitfalls in pertussis serology is drawing blood too early. During the first two weeks of coughing, the immune system has often not had time to generate a measurable antibody spike. Studies have shown that serology performed in that first-two-week window catches only about 13 to 24 percent of infections, a sensitivity so low it is essentially useless as a standalone diagnostic tool.⁸ This is the opposite pattern from culture and PCR testing, which work best early and lose sensitivity as the weeks pass.

The sweet spot for serology starts after two weeks of cough. Most pertussis patients reach diagnostic IgG levels within about four weeks of symptom onset.¹ This makes the test especially valuable in adults and adolescents who have been coughing for weeks before anyone thinks to check for pertussis. In one study of adults hospitalized with prolonged cough lasting two or more weeks, about 7 percent had anti-PT IgG levels at or above 100 IU/mL, compared to just 1 percent of controls without cough.⁹

If your doctor wants the most definitive serological evidence, paired samples are ideal: one drawn during the illness and another drawn two to four weeks later. A threefold or greater rise in IgG-PT between the two samples is highly specific for active infection, with sensitivity and specificity both above 99 percent in large analyses.⁶ In practice, though, paired samples are often impractical. Most people do not have a baseline blood draw sitting on file, and by the time pertussis is suspected, the cough has already been dragging on for weeks. That is why the single-sample high-value threshold exists.

Can Vaccination Explain a High Result?

This is one of the most common questions people have after seeing an elevated result: could it just be from my Tdap shot? The short answer is that vaccination rarely pushes anti-PT IgG above the diagnostic cutoff for infection, but the timing matters.

After a Tdap booster, IgG levels begin climbing within about five to seven days and approach their peak around day 14.¹⁰ The peak levels after vaccination, however, tend to be substantially lower than those seen after natural infection. Modeling studies comparing vaccine responses head-to-head with natural infection found that PT-specific antibody peaks after vaccination are smaller and decay slightly faster than those triggered by actual B. pertussis infection.¹¹ After a Tdap booster, serum IgG can remain elevated for up to about 24 months, but the levels typically sit well below the 100 IU/mL infection threshold for most of that period.¹²

There is a narrow window, roughly one to four weeks post-vaccination, during which an antibody test could be confusing. If you happened to get your blood drawn at the exact peak of your booster response, the level might overlap with the lower end of the infection range. This is why clinicians are generally advised not to rely on single-sample serology in someone who has received a pertussis-containing vaccine within the past year. If your doctor knows about a recent vaccination, they will interpret the number accordingly or may prefer PCR testing instead.

The antibody profiles also differ qualitatively between infection and vaccination, though this is not something a standard clinical lab test distinguishes. Natural infection produces higher levels of antibodies that recognize specific protective regions of pertussis toxin, while acellular vaccines tend to elicit antibodies against the detoxified form of the toxin used in the vaccine.¹³ These distinctions are research tools, not routine clinical tests, but they underscore that the immune system responds to the real bacterium differently than to the vaccine.

How Quickly Do Antibodies Fade?

Once anti-PT IgG peaks, it does not stay elevated indefinitely, which is actually helpful for interpretation. If your level is sky-high, the infection is probably recent. If it is only moderately elevated, it could be weeks to months old. Researchers who followed pertussis patients over time found that the diagnostic-level spike is temporary, with considerable variation between individuals in both the peak reached and the speed of decline.¹⁴

In adults tracked over roughly two and a half years after pertussis, antibodies to PT decreased faster than antibodies directed at other B. pertussis proteins like FHA, pertactin, and fimbriae.¹⁵ There was a tendency for older adults to mount a quicker, higher peak followed by a faster drop compared to younger patients, though these age-related differences were not dramatic.¹⁶ The general pattern is that within several months, levels drift back below the diagnostic cutoff, and over years they return toward baseline. This decay timeline is part of why the 100 IU/mL threshold works: if the level is that high, the exposure was probably within the past few months.

High Antibodies Without Classic Whooping Cough

One finding that surprises many people is that you can have diagnostically elevated pertussis antibodies without ever having the stereotypical whoop or violent coughing spells. Subclinical and mild pertussis infections are far more common than the classic textbook presentation suggests, especially in people who were previously vaccinated or previously infected.

Household contact studies have found that when a child is diagnosed with pertussis, about two-thirds of infected family members who were previously immunized had subclinical infections with no recognizable symptoms, yet all of them showed diagnostically elevated pertussis antibodies at the time of testing.¹⁷ Population-level serosurveys reinforce this picture. In one study, nearly 6 percent of people aged 1 to 35 had high IgG titers suggestive of recent pertussis infection, many of whom had no history of a recognized cough illness.¹⁸

Asymptomatic infections and carriage appear to be widespread among vaccinated populations.¹⁹ So if your blood test comes back with a high anti-PT IgG and you think “but I wasn’t that sick,” the result is not necessarily wrong. You may have had a mild or silent infection that your immune system handled without producing the full-blown whooping cough picture.

Why Labs Report Different Numbers

If you have been tested at different labs or compare your result to numbers cited in medical literature, you may notice the values do not line up neatly. Part of the problem is assay standardization. For years, different commercial ELISA kits used different antigens, different calibration standards, and different units, making results hard to compare across labs.

European reference laboratories have pushed for standardization around a few principles: the ELISA should use purified, non-detoxified pertussis toxin as the antigen, results should be expressed in international units per milliliter (IU/mL), and assays should be calibrated against a shared reference preparation.²⁰ The World Health Organization has prepared candidate international reference standards to facilitate this, but adoption across all commercial kits has been uneven.²¹ Kits that use purified PT as the antigen show reasonable agreement with the WHO reference preparation, while kits using mixed antigens do not.²²

The practical takeaway is that you should interpret your result against the reference range printed on your specific lab report, not against a number you found online. A result of “80 IU/mL” from one kit may not mean the same thing as “80 IU/mL” from another. If your clinician is uncertain, the lab performing the test can usually clarify what cutoffs they use and which antigen their assay targets.

The Role of Your Vaccine History

Your original childhood pertussis vaccine series can shape how your immune system responds to both infection and booster shots years later, and this can subtly affect antibody levels. People who received the older whole-cell pertussis vaccine as infants tend to mount higher anti-PT IgG responses after a booster dose in adolescence compared to those primed with the newer acellular vaccine.²³ These differences in immune memory can persist into adulthood, with the two groups showing distinct antibody profiles even before a booster is given.²⁴

Acellular pertussis vaccines also tend to lose effectiveness faster, with waning protection observed as early as two to three years after booster doses.²⁵ After a primary acellular series in infancy, antibody levels to PT, FHA, and pertactin dropped steeply and were near the limit of detection within 15 months.²⁶ This rapid waning is one reason why so many vaccinated adults and adolescents end up susceptible to B. pertussis and can develop the elevated antibodies that come with natural infection.

Pertussis Antibodies in Pregnancy and Newborns

If you are pregnant or recently had a baby and see pertussis antibody results for yourself or your infant, the interpretation is a bit different. Maternal Tdap vaccination during pregnancy is recommended specifically because anti-PT IgG crosses the placenta efficiently. In one study, the transplacental transfer ratio for PT-specific IgG was about 1.35, meaning cord blood actually had higher antibody concentrations than the mother’s blood at delivery.²⁷

These passively transferred antibodies decline quickly in the infant. The half-life of PT-specific IgG in newborns is roughly 29 days, meaning the level halves every month. By six weeks of age, the proportion of infants with anti-PT levels above 10 IU/mL dropped from 97 percent at birth to 67 percent.²⁷ Another study found that by two months of age, about 66 percent of infants still had anti-PT levels at or above 10 IU/mL.²⁸ So a high anti-PT IgG in a newborn reflects the mother’s immune status, not the baby’s own response. In a very young infant, the number tells you about passive protection; it does not indicate the baby has been infected.

Premature birth complicates this further. Transfer rates for both anti-PT and anti-FHA antibodies are significantly lower in babies born before 32 weeks of gestation compared to term infants.²⁹ A preterm baby may therefore have lower antibody levels at birth even if the mother was well-vaccinated, and those levels will drop below any meaningful threshold sooner.

Pertussis in Older Adults

Pertussis is often thought of as a childhood disease, but it circulates actively in older adults, many of whom are no longer protected by vaccines received decades earlier. A community study of elderly residents found that the rate of serologically defined B. pertussis infection was about 3.3 per 100 person-years when using strict criteria (a twofold or greater rise in anti-PT antibodies) and 8.0 per 100 person-years using slightly broader criteria.³⁰ Only about half of those with definite infection had recognizable respiratory symptoms at the time. The study also noted that antibody to FHA, one of the non-PT antigens, seemed to get boosted by infections with other organisms, not just pertussis. This is a reminder that in older adults, only anti-PT IgG should be used for diagnostic purposes; elevated antibodies against other pertussis proteins may reflect unrelated respiratory infections.

What to Actually Do With the Result

If your lab result shows anti-PT IgG at or above 100 IU/mL and you have had a cough for more than two weeks, the result strongly supports a diagnosis of pertussis, regardless of whether a culture or PCR test was ever done. By the time serology is positive, you are usually past the window where antibiotics can shorten the illness for you, but treatment with a macrolide antibiotic may still be prescribed to reduce the chance of spreading the bacteria to vulnerable contacts, particularly unvaccinated infants.

If the level is elevated but falls below the diagnostic cutoff, your doctor may order a second sample in two to four weeks to look for a rise. A threefold increase between samples is considered highly specific for active infection. Alternatively, clinical context may be enough: a prolonged cough illness in someone with a moderately elevated anti-PT IgG and a known pertussis exposure is, for practical purposes, pertussis.

One thing to keep in mind is that serology does not tell you whether you are still infectious. Pertussis is most contagious during the first couple of weeks of coughing, before antibodies have even risen. By the time your IgG is high enough to be diagnostic, you have likely already passed the most contagious period. Public health authorities still want to know about confirmed or probable cases, because contact tracing can protect people around you who may still be in their early, most transmissible phase of infection after catching it from you.