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CDC Researchers Find Low Sensitivity for Rapid Flu Tests, Urge Use of rRT-PCR

NEW YORK (GenomeWeb News) – The sensitivity of rapid influenza diagnostic tests for detecting the new H1N1 influenza A ("swine flu") and other flu viruses drops off in patient samples containing lower viral titer, according to this week's issue of the US Centers of Disease Control and Prevention publication Morbidity and Mortality Week Report.

CDC researchers compared rapid influenza diagnostic tests from three companies using 65 clinical samples (nasopharyngeal and oropharyngeal swabs) collected in April and May. All of these samples had previously tested positive for a flu virus — either the new H1N1 influenza A strain or a seasonal H1N1 or H3N2 influenza A strain — using the CDC-developed, US Food and Drug Administration approved real-time reverse transcription PCR test.

The researchers re-tested these samples using the Inverness Medical BinaxNOW Influenza A&B test, Becton Dickinson's Directigen EZ Flu A+B test, and Quidel's QuickVue Influenza A+B test. They also tested a subset of the samples using other rapid diagnostic tests from four other companies, though those results were not reported in MMWR.

The team found that the rapid influenza tests could detect the swine flu and other viruses — but the sensitivity was only high when samples contained the highest concentration of flu virus. One test accurately detected all nine cases with the highest viral titer, while the other two found eight of the nine cases.

The sensitivity dropped off for all three tests at lower viral titer, the researchers reported. And the overall sensitivity was roughly 40 to 69 percent. The BinaxNow test positively identified 40 percent of the flu cases, the Directigen EZ test found 49 percent, and the QuickVue test had 69 percent sensitivity.

Based on these results, the researchers noted that "although a positive [rapid influenza diagnostic test] result can be used in making treatment decisions, a negative result does not rule out infection with a novel influenza A (H1N1) virus." Consequently, they recommended taking into account other clinical and epidemiological factors when treating patients with negative rapid influenza diagnostic test results.

"Because of the limitations of [rapid influenza diagnostic tests] and until additional data are available, all results from [rapid influenza diagnostic tests], both positive and negative, when used for clinical decision-making in a patient with suspected novel influenza A (H1N1) virus infection, should be interpreted in the context of circulating influenza strains in the patient's community, the level of clinical suspicion, severity of illness, and risk of complications," the authors concluded.

The researchers encouraged the use of rRT-PCR testing in situations where "a more definitive determination of infection with influenza virus is required."

Along with the rRT-PCR test developed by the CDC, Quest Diagnostics was granted an emergency authorization from FDA for its rRT-PCR-based swine flu test last month. Meanwhile, firms such as Prodesse are marketing their own real-time PCR tests for detecting influenza strains, in general. In June, the FDA sent warning letters to Prodesse, BD, and Luminex, cautioning them against marketing their influenza diagnostic tests as tests for detecting the new H1N1 strain.

The CDC group noted that the current study is preliminary and subject to limitations — including the fact that the samples they tested had been stored and shipped to the CDC rather than being tested right after they were collected.

They also urged other researchers to do their own evaluations of rapid flu diagnostic tests. A few such studies have already been conducted, they noted, including two that put the sensitivity of rapid influenza diagnostic tests at between 10 and 51 percent for detecting the new H1N1 strain.

For instance, earlier this year, North Shore-Long Island Jewish Health System's Director of Virology and Molecular Diagnostics Christine Ginocchio led a team of researchers who compared the sensitivity and specificity of various diagnostic approaches including the BinaxNOW Influenza A&B test, the 3M Rapid Detection Flu A +B test, direct immunofluorescence, R-Mix culture, and the Luminex xTAG Respiratory Virus Panel approaches all detected H1N1.

In that study, published in the Journal of Clinical Virology in July, researchers concluded that Luminex's Respiratory Virus Panel, which had nearly 98 percent sensitivity in that study, "provided the best diagnostic option" of the methods tested for detecting flu strains, including H1N1. In the same study, the sensitivity of rapid antigen tests was less than 18 percent.

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