Rapid diagnosis of influenza A infection by direct immunofluorescence of nasopharyngeal aspirates in adults

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Abstract
The efficacy for direct immunofluorescence of a commercial conjugate for influenza A virus prepared against whole A/Udorn (H3N2) virus was studied. The conjugate was specific for influenza A virus, but its sensitivity varied depending upon the strain of influenza A tested. Nasopharyngeal aspirates collected from 25 patients during an outbreak of influenza were examined for viral antigen with the conjugate and inoculated onto rhesus monkey kidney (MK) cells for virus isolation. Patients (15) had isolates for influenza A/USSR/90/77 (H1N1); nasopharyngeal secretions werefluorescence antibody [Ab] positive in 12. Fluorescent Ab was co-positive with culture in 11/15 patients (73.3%) and co-negative in 9/10 (90%). The 1 fluorescent Ab-positive, culture-negative patient had negative serology for influenza A and the fluorescent Ab result was considered a false positive. At a 1:10 dilution, the conjugate stained nasopharyngeal and MK cells infected with A/USSR (H1N1) 2 to 3+, whereas the cells infected with H3N2 virus stained 4+. A conjugate made specifically against the ribonucleoprotein antigen, which is universal to all influenza A strains, may improve the sensitivity of the direct immunofluorescent test.