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Influenza A Virus pH1N1 Often Not Detected by Rapid Flu Tests

CDC has received a number of reports of severe respiratory illness among young and middle-aged adults that have required admission to the ICU. Some fatalities have been reported.  Laboratory surveillance data as shown that the majority of cases have been caused by the 2009 strain of pandemic influenza A virus pH1N1.

Physicians often confirm their suspicion of influenza with a rapid test for Influenza, especially early in the flu season. Rapid flu tests are qualitative immunoassays that can identify the presence of influenza A and B viral nucleoprotein antigens in respiratory specimens in approximately 15 minutes or less.. Testing specimens within the first 48 to 72 hours after onset of illness, when viral shedding is highest, is most likely to yield a positive result.  However, rapid flu tests have limited sensitivity and are dependent on the adequacy of the specimen. A negative test result does not exclude influenza virus infection in patients with signs and symptoms suggestive of influenza. Antiviral treatment should not be withheld from patients with suspected influenza, even if they test negative with a rapid flu test.

The most sensitive test for laboratory confirmation of influenza virus infection is reverse transcription-polymerase chain reaction. These tests are available from diagnostic companies such as Cepheid’s XPert Flu and Biofire Diagnostics Respiratory virus panel.

 Testing is not needed for all patients with signs and symptoms of influenza to make antiviral treatment decisions. Once influenza activity has been documented in the community or geographic area, a clinical diagnosis of influenza can be made for outpatients with signs and symptoms consistent with suspected influenza, especially during periods of peak influenza activity in the community. .



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