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 for accurate diagnosis 1. In the early stages of whooping cough (pertussis), antibody tests may produce false negative results because the body hasn't yet produced sufficient antibodies against Bordetella pertussis. For this reason, PCR testing of nasopharyngeal specimens is preferred during the first 3 weeks of illness when the bacteria are still present in the respiratory tract. After this initial period, as the infection progresses, antibody testing becomes more useful for diagnosis. Some key points to consider in the diagnosis of pertussis include:
- The clinical case definition, which includes an acute illness with a cough that is persistent for > 2 weeks and is associated with posttussive vomiting, the typical whooping sound, or severe paroxysms 1.
- The use of laboratory methods, such as culture, polymerase chain reaction (PCR), serology, and oral fluid testing, to confirm a diagnosis of pertussis, with specificities ranging from 72% to 100% 1.
- The importance of clinical judgment in diagnosis, particularly in adults and children, as reflected in the clinical definitions used by the World Health Organization, Centers for Disease Control and Prevention (CDC), and Public Health England 1. IgG antibodies against pertussis toxin are the most specific marker and can remain elevated for months to years after infection. However, interpretation can be complicated by previous vaccination, which also produces antibodies. For optimal diagnosis, the timing of testing should be considered alongside clinical symptoms, which typically progress from initial cold-like symptoms to the characteristic paroxysmal coughing fits that can last for weeks to months. It's worth noting that the most recent guideline from 2019 focuses on making the clinical diagnosis of pertussis-associated cough in adults and children, highlighting the importance of clinical judgment in diagnosis 1.
From the Research
Antibody Detection in Pertussis
- Antibodies against pertussis can be detected through various methods, including enzyme immunoassay (EIA) 2.
- 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 and IgA antibodies are detected later 2, 3.
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 from week 1 to 3 after the onset of the disease, with 67% and 78% of patients having increased levels, respectively 2.
- The combination of IgA, IgG, and IgM antibody levels is the most effective approach for serological diagnostics of pertussis due to the heterogeneity of anti-pertussis antibodies at different stages after the onset of the disease 2.
Sensitivity and Specificity of Antibody Detection
- The sensitivity and specificity of single IgA and IgG levels for early diagnosis of pertussis in adults have been assessed, with levels of 24 U/ml for IgA and 27 U/ml for IgG giving sensitivities of 100% and 75%, respectively, in the first 2 weeks 3.
- The specificity of IgA and IgG levels was 82% and 89%, respectively, in internal controls, and 90% in external controls 3.
- The diagnostic utility of in-house anti-PT IgG and anti-PT IgA assays has been evaluated prospectively in an Australian community-based cohort, with both assays having good specificity, but anti-PT IgG assay being consistently more sensitive than anti-PT IgA assay 4.