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Many respiratory pathogens can produce an influenza-like illness. With a sensitive and specific rapid test for influenza, the onset of outbreaks could be established and patients could receive appropriate antiviral treatment. Reverse-transcriptase PCR (RT-PCR) can yield results rapidly and is potentially the most sensitive test, but delays associated with shipping of specimens and laboratory-based testing limit the real-time value of this test. Although widely used in emergency departments and physicians' offices, antigen-based rapid assays have shown a sensitivity of about 50% compared with culture or molecular diagnostic techniques.
Investigators in Germany recently compared the sensitivity of a commercially available antigen-based rapid test (BinaxNOW Influenza A & B Rapid Test) with that of a real-time RT-PCR assay specific for the hemagglutinin gene of the 2009 H1N1 virus. Of 1838 clinical specimens tested, 221 were confirmed as H1N1 positive by RT-PCR. When 144 of these PCR-positive specimens were evaluated using the rapid-antigen test, results were positive for only 16 (11.1%).
Drexler JF et al. Poor clinical sensitivity of rapid antigen test for influenza A pandemic (H1N1) 2009 virus. Emerg Infect Dis 2009 Oct; 15:1662.
Comment
There's a lot of bad news here on influenza testing. Although the authors found only 11% sensitivity of the rapid test compared with PCR, the “gold standard” PCR detected the 2009 H1N1 virus in just 12% of 1838 respiratory specimens submitted at a time when this virus was pandemic and probably responsible for most influenza-like illness. The relative sensitivity of the rapid test may have been diminished by the use of frozen-thawed specimens, when the test is intended for use with fresh material. Although we don't know how other, similar assays would have performed, rapid tests appear to be unsuitable for use among patients suspected of having 2009 H1N1 influenza. Clearly, better rapid tests are needed.