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Influenza Viruses

Influenza Viruses

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Influenza Viruses

Laboratory testing of patients with respiratory illness can be helpful in identifying the cause of the illness. When that cause is an influenza virus, testing can differentiate the various types of influenza. There are 3 types of influenza viruses that affect humans: A, B, and C. Type A viruses occur in humans and animals, are classified by subtype, and can cause both epidemics and pandemics. Type B viruses occur primarily in humans, are not subtyped, and can cause epidemics but not pandemics. Type C viruses are associated with milder illness and are not associated with either epidemics or pandemics.

Type A virus classification is based on hemagglutinin (HA) and neuraminidase (NA) surface glycoproteins. There are 16 known HA and 9 known NA subtypes. So, the type A H1N1 virus is comprised of an HA 1 glycoprotein and an NA 1 glycoprotein. Types A H1N2 and H3N2 are common causes of the "seasonal" flu that typically occurs in the autumn and winter months. The emergence of the 2009 H1N1 influenza virus (previously known as the "swine flu" virus) resulted in the first influenza pandemic in more than 40 years. It is now a relatively common cause of seasonal influenza. The influenza A H5N1 virus (the "avian" or "bird" flu) is a rare, but serious, cause of illness in people in some parts of the world who have been in direct contact with infected birds.

Multiple laboratory test methods are available for influenza virus testing. Polymerase chain reaction (PCR)-based tests can differentiate influenza A from B and specifically identify the 2009 H1N1 virus. PCR provides results more rapidly than conventional culture and is the most sensitive routine testing method. Antigen detection by immunoassay or direct fluorescent antibody (DFA) can distinguish influenza A from B (sensitivity <70% for immunoassay and 70% to 80% for DFA) and can typically be performed within 1 day. Combining rapid culture with DFA can detect and distinguish influenza A from B (sensitivity >95%) in less than 24 hours of sample arrival in the lab. However, the rapid culture method cannot distinguish type A subtypes. In guidelines released in October 2009, the American Society for Microbiology stated that "Viral cell culture or PCR-based assays are the preferred methods of testing."1

Antibody tests may not detect current infection, and those using complement fixation (CF) are not suitable for determining vaccine response.


  1. American Society for Microbiology. ASM's interim algorithm for guidance in testing of patients with respiratory illness for influenza A (including novel H1N1). http://www.asm.org/images/pdf/Flu/fluasmalgorithm-2.pdf. Accessed February 11, 2015.

 Content reviewed 02/2015


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