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BACKGROUND: Serodiagnosis for pertussis through commercially-available assays is increasingly being used in the US despite the lack of information available about their performance and clinical relevance. To gain a better understanding of the utility of these assays in diagnosing pertussis, commercially-available kits were initially tested with a smaller panel (n=20) of sera to assess analytical characteristics, then later with a larger panel (n=226) to assess clinical sensitivity and specificity.
METHODS: Forty-five antigen-antibody combinations were evaluated. Serum panels included culture and/or PCR-confirmed clinically-suspect cases, as well as healthy donor sera. The WHO International Reference Standard was included as a control in the first panel to evaluate accuracy of the assay. The first panel was tested in a 2x2x2 matrix with two dilutions/assay and 2 assays/day performed by two analysts to assess precision. The second panel was run once per commercial assay. Analytical analyses included coefficients of variation (CV), concordance correlation coefficients (rc), and accuracy.
RESULTS: Intra-analyst variability was found to be relatively low among samples per assay, with only 7% (80/1126) having CV > 20%, primarily with the highly concentrated IgG anti-pertussis toxin (PT) specimens and IgM assays. rc measurements ranged between 0.362-0.972, 0.332-0.998, and 0.007-0.470 for IgA, IgG, and IgM, respectively. Analytical accuracy was 86-115% for the four IgG anti-PT assays that were calibrated to a reference serum at the time of testing. In the clinical assessment, sensitivity and specificity varied greatly for all assays; ranges of % sensitivity/specificity for IgA, IgG, and IgM assays were 39-89/16-100, 61-96/31-100, and 0-10/90-100, respectively. For most assays, adding filamentous hemagglutinin (FHA) with PT increased sensitivity, but lowered specificity.
CONCLUSIONS: Commercially-available seroassays varied substantially under both analytical and clinical parameters; however, IgG anti-PT assays that were calibrated to a reference standard were highly accurate. Our findings support incorporation of calibrated pertussis seroassays to the pertussis case definition for improved diagnosis and surveillance.