A high result on a Bordetella pertussis IgG antibody test indicates that your immune system has been activated to fight the bacterium that causes whooping cough. Bordetella pertussis is a highly contagious respiratory bacterium known for causing severe, prolonged coughing fits. The test measures the presence and quantity of Immunoglobulin G (IgG) antibodies, which are specialized proteins produced in response to exposure to the bacterium or the vaccine. A high result suggests you have immune memory related to whooping cough, but understanding the exact context requires considering your personal health history.
Understanding the IgG Antibody Test for Pertussis
The purpose of an antibody test is to determine if the body has mounted an immune response to a specific pathogen. Antibodies function as the immune system’s long-term memory, recognizing and neutralizing foreign threats. Immunoglobulin G (IgG) is the most common type of antibody found in blood and provides enduring protection. IgG levels typically become detectable about two weeks after the body is first exposed to the bacterium or vaccine.
The test specifically targets antibodies against Pertussis Toxin (PT), which is a poison produced only by the Bordetella pertussis bacterium. Because PT is unique to this bacterium, antibodies against it are a highly specific marker for exposure, whether through natural infection or vaccination. This IgG marker will remain in the bloodstream long after the illness or vaccination has occurred, providing a picture of your past immune events. Clinicians often contrast the IgG result with Immunoglobulin M (IgM) and IgA, which are typically earlier markers that suggest a more recent or ongoing infection.
Differentiating Causes of a High IgG Result
An elevated IgG antibody titer, often defined as a concentration of 100 IU/mL or greater, indicates a robust immune response to the Pertussis Toxin. The two primary reasons for this high level are a recent vaccination or a past, possibly unrecognized, natural infection. Differentiating between these two causes is challenging because the vaccine contains the Pertussis Toxin antigen, eliciting the same IgG response as the bacterium itself.
One way to interpret the result is by considering the timing of the last Tdap vaccination, as a booster shot within the last two to six months can cause a significant, temporary spike in IgG levels. If the patient has been symptomatic for two or more weeks and has not had a recent vaccination, a high IgG result is more likely to suggest a recent infection. For a definitive diagnosis of current infection, a healthcare provider may look for a four-fold or greater rise in the IgG level between a sample taken early in the illness and a second sample taken two to four weeks later.
The high level itself does not automatically confirm a current, active illness, but suggests recent exposure that prompted a strong immune reaction. In the absence of a recent vaccine, an IgG level above the established threshold is considered diagnostic of recent or active Bordetella pertussis infection. Clinicians must combine the numerical result with the patient’s symptoms, age, and vaccination history to form an accurate conclusion.
Clinical Implications and Next Steps
A high B. pertussis IgG result suggests you have protective antibodies, but this protection is not permanent and wanes over time, necessitating booster shots. After an infection, the high IgG level will gradually decline, often decreasing to lower levels within 12 to 23 months.
If the high IgG result is obtained while you are experiencing persistent, severe coughing, it supports a diagnosis of whooping cough in the later stages of the illness. At this point, other tests like Polymerase Chain Reaction (PCR) are often less effective, making serology useful for delayed diagnosis. The Centers for Disease Control and Prevention generally recommend PCR or culture as first-line tests for people symptomatic for less than two weeks, but serology remains useful for prolonged cough illnesses.
A healthcare provider may order supplementary diagnostic tests, such as IgM or IgA antibodies, or a PCR test, if they suspect a current infection alongside the high IgG. This result provides valuable information about your immune status and history of exposure, and should be discussed with a doctor for personalized guidance regarding treatment or future vaccination planning.