From the Guidelines
Pertussis antibody testing typically becomes reliable 2-3 weeks after symptom onset, as indicated by the need for paired sera samples taken within 2 weeks of onset and 3 to 4 weeks later to demonstrate a significant increase in IgG serum antibody against PT 1. In the early stages of whooping cough (pertussis), antibody tests may produce false negative results because the body hasn't had sufficient time to develop a detectable immune response. For this reason, PCR testing of nasopharyngeal specimens is preferred during the first 3-4 weeks of illness when the bacteria (Bordetella pertussis) is still present in the respiratory tract. After this initial period, as bacterial DNA decreases, antibody testing becomes more useful for diagnosis. Specifically, tests measuring IgG antibodies against pertussis toxin are most reliable after the third week of illness, as supported by the most recent guideline on clinically diagnosing pertussis-associated cough in adults and children 1. This timing reflects the natural immune response to infection, where antibody production follows the initial bacterial colonization and symptom development. Key points to consider include:
- The importance of clinical judgment in diagnosis, as reflected in the clinical definitions used by the World Health Organization, Centers for Disease Control and Prevention (CDC), and Public Health England 1
- The role of laboratory methods, such as culture, polymerase chain reaction, serology, and oral fluid testing, in confirming a diagnosis of pertussis 1
- The need for early antibiotic therapy, typically azithromycin for 5 days, to reduce transmission, even if it may not significantly alter the course of established disease 1.
From the Research
Antibody Detection in Pertussis
- Antibodies against pertussis can be detected through various methods, including enzyme immunoassay (EIA) and enzyme-linked immunosorbent assay (ELISA) 2, 3, 4, 5.
- The detection of antibodies depends on the type of antibody and the stage of the disease. For example, IgM antibodies are typically detected early in the disease, while IgG antibodies are detected later 2, 5.
Timing of Antibody Detection
- Increased levels of IgM antibodies can be detected predominantly from week 1 to 5 after the onset of the disease 2.
- IgA and IgG antibodies can be detected as early as 1-2 weeks after the onset of symptoms, with levels reaching a peak around 2-3 weeks after onset 3, 5.
- In some cases, antibody levels can remain elevated for several months after the onset of symptoms, making it possible to detect antibodies even after the acute phase of the disease has passed 3, 4.
Sensitivity and Specificity of Antibody Detection
- The sensitivity and specificity of antibody detection can vary depending on the method used and the population being tested 3, 5, 6.
- A study found that a single IgA level of 24 U/ml and a single IgG level of 27 U/ml could be used as sensitive and specific thresholds for diagnosing pertussis in adults 3.
- Another study found that a combination of IgG anti-pertussis toxin and IgA anti-filamentous hemagglutinin using age-specific reference values had a sensitivity of 81-89% in diagnosing pertussis from a single serum sample taken 5-10 weeks after the beginning of symptoms 5.