Differential diagnosis of pandemic (H1N1) 2009 infection by detection of haemagglutinin with an enzyme-linked immunoassay

A sensitive and convenient immunoassay that can directly differentiate pandemic (H1N1) 2009 (pH1N1) virus from seasonal influenza virus can play an important role in the clinic. In the presented study, a double-sandwich ELISA (pH1N1 ELISA), based on two monoclonal antibodies against haemagglutinin (HA) of the pH1N1 virus, was developed. After laboratory determination of the sensitivity and specificity characteristics, the performance of this assay was evaluated in a cohort of 904 patients with influenza-like illness. All seven strains of pH1N1 virus tested were positive by pH1N1 ELISA, with an average lower detection limit of 103.0 ± 0.4 tissue culture infective dose (TCID)50/mL (or 0.009 ± 0.005 HA titre). Cross-reaction of the assay with seasonal influenza virus and other common respiratory pathogens was rare. In pH1N1-infected patients, the sensitivity of the pH1N1 ELISA was 92.3% (84/91, 95% CI 84.8–96.9%), which is significantly higher than that of the BD Directigen EZ Flu A + B test (70.3%, p <0.01). The specificity of pH1N1 ELISA in seasonal influenza A patients was 100.0% (171/171, 95% CI 97.9–100.0%), similar to that in non-influenza A patients (640/642, 99.7%, 95% CI 98.9–100.0%). The positive predictive value for pH1N1 ELISA was 97.7% and the negative predictive value was 99.1% in this study population with a pH1N1 prevalence of 10.1%. In conclusion, detection of HA of pH1N1 virus by immunoassay appears to be a convenient and reliable method for the differential diagnosis of pH1N1 from other respiratory pathogens, including seasonal influenza virus.