Rapid tests for swine flu are not very accurate

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Rapid tests used to diagnose pandemic H1N1 influenza infection in a doctor’s office are, in general, accurate no more than half the time, according to new research by the Centers for Disease Control and Prevention. If the test comes back positive, you most likely do have the virus, commonly known as swine flu, but if the test is negative, the results are probably meaningless, the researchers reported today in the CDC’s Morbidity and Mortality Weekly Report.

The researchers are not talking about the sophisticated laboratory tests used by CDC and other public health authorities to detect swine flu virus. Those tests look for genetic material from the virus and are considered highly accurate. The rapid tests used in a doctor’s office are more like home pregnancy tests. They use an antibody against the virus and provide a quick visual indication of its presence. Because of that, they are more accurate when large amounts of virus are present, in the first day or two of infection. The rapid tests also detect flu viruses in general, and are not specific for swine flu.

The researchers studied tests from three companies: Binax Inc. of Scarborough, Maine; Becton, Dickinson & Co. of Sparks, Md.; and Quidel Corp. of San Diego. They used clinical specimens that had been shown to contain an influenza virus through laboratory testing. When they looked at nine specimens that had large amounts of virus, the Quidel test detected all nine, while the other two detected eight. But when they studied specimens with lower levels of virus, the results were much worse. Overall, the Quidel test was the best, detecting 69% of the infections. The Becton, Dickinson test detected 49% and the Binax test detected 40%. Detection levels were higher for seasonal flu than they were for pandemic H1N1.

The new results are not unexpected. In a recent study in the Journal of Clinical Virology, researchers found that one test detected only 10% of swine flu infections, while a second detected only 40%. And in a letter to the New England Journal of Medicine in June, Navy researchers said that the Quidel test detected only half the infections.

Health authorities fear that the negative tests will prevent physicians from treating patients with antiviral drugs, particularly the patients at high risk, such as pregnant women and those with asthma and other underlying medical conditions.

-- Thomas H. Maugh II