Influenza, Influenza-Associated Mortality, and Novel Influenza A
Laboratory Procedures
- Polymerase-chain Reaction (PCR): PCR is the most sensitive method for the detection of influenza viruses and the gold standard for influenza diagnosis. PCR tests can detect influenza virus genomes even when they are present at very low levels and can provide rapid identification of viruses.
- Viral Culture: Viral isolation is essential for influenza surveillance. Viral culture testing may take up to 3 weeks for strain identification; therefore it is not suitable for diagnosis in determining treatment options. Virus isolation has the advantage of producing quantities of virus sufficient for full antigenic characterization, which is required for determining vaccine match, and conducting testing for antiviral resistance.
- Rapid Influenza Diagnostic Test (RIDT): RIDTs test for the presence of viral antigens but are less sensitive than PCR testing. RIDTs can produce quick results in less than 30 minutes and are simple to perform. The sensitivity and specificity of these rapid tests is usually dependent on the type of test, the quality of specimen collected, the time it was collected and on the amount of virus collected in the specimen. Currently available RIDTs fall into two groups that can either detect the presence of an influenza A or B virus but cannot differentiate between the two, or detect both influenza type A and B viruses and distinguish between the two. No RIDT can currently provide information on virus subtypes or strains.
- Serology: Serology may be used for laboratory confirmation of influenza, but is not the recommended procedure for routine diagnosis. If a unique or a new strain (subtype) of influenza virus is suspected, this method may prove valuable. The laboratory diagnosis will be based on a four-fold rise in antibody titer between the acute and convalescent specimens of serum.
- Immunofluorescent Antibody Staining: Cells from respiratory specimens can be stained using an immunofluorescent antibody that reveals the presence of viral antigen using a fluorescent microscope.
- Immunohistochemical (IHC) Staining: IHC staining for influenza viral antigens can be performed in respiratory tract tissue.