Etiological diagnosis of influenza A virus by enzymatic radioimmunoassay

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Abstract
An enzymatic radioimmunoassay for influenza A virus was developed by using polystyrene beads coated with rabbit IgG to capture viral hemagglutinins (H1 and H3). Captured hemagglutinin was detected with goat IgG followed by affinity-purified rabbit anti-goat IgG labeled with alkaline phosphatase. [3H]AMP was added to quantify alkaline phosphatase activity; free [3H]adenosine was measured with a scintillation counter. The assay detected as little as 0.1 ng of purified hemagglutinin. It was specific for hemagglutinin subtype and, depending on the source of the goat IgG used, detected H1 or H3. There was no reaction with neuraminidase or core antigens of influenza strain WSN-33. The clinical efficacy of the assay was evaluated with sequential nasal washes from 33 patients with naturally acquired H1N1 influenza. In the first 3 days of infection, the assay was consistently less sensitive than the viral culture, although detectable antigen persisted in secretions longer than did the infectious virus. Testing of multiple samples greatly increased the number of individuals in whom an etiological diagnosis could be made by immunoassay (81% of patients were positive for viral antigens at some point in their illness); such testing was necessary to achieve the sensitivity of a single culture. Mean antigen levels were highest in nasal washes with the highest titers of infectious virus.

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