Rapid diagnosis of influenza A infection by immunofluorescence. Methodological problems and clinical material.

Indirect immunofluorescence (IF) on cell spreads from nasopharyngeal secretions (NPS) was used for the rapid diagnosis of influenza A infection and was compared with IF on monkey kidney cells infected by NPS. The clinical diagnosis of influenza A infection was confirmed by serology in 32 of 40 patients. In 27 of the 32 patients (84 per cent), the diagnosis was achieved by IF on cell spreads of NPS. In 13 of 15 (87 per cent) subjects with positive serology, the early appearance of influenza A virus antigen was revealed by indirect IF on infected monolayer cells. No false positive specimens were found among serologically negative subjects by either method. Consequently, the reliability of IF on cell spreads of NPS is very similar to IF on infected cell cultures, but offers a much quicker diagnosis (3-4 h as compared to 1-3 d). Monovalent anti-human IgG FITC should be used instead of polyvalent anti-human Ig FITC, as the latter contains anti-IgA which may adhere to intracellular IgA in the epithelial cells of NPS and thereby cloak the viral antigen.