Nasopharyngeal pertussis toxin IgA antibodies in the diagnosis of pertussis in Australian community patients

Nasopharyngeal aspirate (NPA) Bordetella pertussis-specific IgA antibody assay using whole-cell (WC) antigen has previously been shown to have promise in the diagnosis of patients with suspected pertussis. Recently, the use of WC assays in serum have been replaced by pertussis toxin (PT) because of specificity concerns. In this study, PT and WC B. pertussis-specific IgA antibody was assayed in 491 NPAs. Specimens also had molecular testing for the presence of B. pertussis and B. parapertussis as per the usual laboratory protocol. Positive concordance of the two serological assays was 51.2%, negative concordance was 67.5% and total concordance was 75.8%. 99 of 119 discordant specimens were resolved by utilising the B. pertussis polymerase chain reaction (PCR) result and clinical status, and yielded a sensitivity of 57.6% and a specificity 97.7% for WC, with 90.2% and 93.1%, respectively, for the PT assay (p < 0.00025 and 0.025–0.01). In contrast, the sensitivity of PCR was only 19.1% in this cohort. We conclude that specificity is not a significant issue for mucosal pertussis-specific IgA assays using WC, but the superior sensitivity of the PT assay favours the latter method. This assay, combined with PCR assays, should significantly improve the diagnosis of pertussis cases.