Virological diagnostics of influenza: detection of infectious agents and assessment of immunity

A variety of methods are available for virological diagnosis and immunity assessment of influenza. For virus detection, the clinical specimen is usually the respiratory swab. In terms of sensitivity, nasopharyngeal aspirates are superior and sputum specimens are inferior. The diagnostic method of choice is RT-PCR of viral DNA sequences coding for matrix and nucleoprotein, which define influenza virus types A, B, C, or coding for haemagglutinin and neuraminidase spikes on the viral envelope identifying influenza virus A subtypes and strains. Genespecific primer probes are commercially available. PCR takes 1-2 h after the arrival of the clinical specimen. Cell culture based virus isolation combined with intracellular antigen detection needs 1-2 days, but is regarded as gold standard. Transport of the swabs in an ice chest is recommendable. Direct antigen detection in the clinical specimen by the use of enzyme-linked immunosorbent assay (ELISA) or immunofluorescence test (IFT) is the most rapid (bedside) method, but of inferior sensitivity. Therapy resistance analysis is done by genotyping subsequent to PCR amplification or by the cell culture method (phenotyping). Antibodies due to influenza virus are produced during the second week after infection and may confirm or disprove virological diagnosis. ELISA and IFT apply nucleoprotein or matrix proteins as antigens and differentiate between the Ig classes IgA (IgM) and IgG indicating an acute or passed infection. Complement fixing antibodies do not persist. Influenza virus immunity is assessed by neutralisation assay or - more simply - by haemagglutination inhibition in a type-, subtype-and even variant-specific manner.