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Rapid Flu Tests Not Always Accurate

Most rapid influenza diagnostic tests (RITDs) perform well when viral concentrations in a sample are high, but accuracy declines at lower concentrations, researchers found.

Of 11 FDA-cleared rapid tests that were commercially available during the 2011-2012 flu season, most detected viral antigens in samples with the highest concentrations, according to Roxanne Shively, MS, of the Department of Health and Human Services (HHS), and colleagues.

Detection varied, however, when the viral concentrations in a sample were lower, the researchers reported in the Nov. 2 issue of Morbidity and Mortality Weekly Report.

And one rapid test -- SAS FluAlert Influenza A from SA Scientific -- did not consistently detect the pandemic H1N1 virus or other influenza A viruses even at high concentrations.

"Clinicians should be aware of the variability of RIDTs when interpreting negative results and should collect test samples using methods that can maximize the concentration of virus antigen in the sample, such as collecting adequate specimens using appropriate methods in the first 24 to 72 hours after illness onset," Shively and colleagues wrote.

"The use of these tests for clinical management and public health practice can be improved by continually updating guidance, educating clinicians on best practices, and enhancing test design for better performance," they wrote, noting that the Joint Commission is offering two online courses dealing with strategies to improve rapid influenza testing in ambulatory settings.

The analysis was conducted as part of a collaboration between the CDC, the Biological Advanced Research and Development Authority at HHS, and the Medical College of Wisconsin. The CDC provided 16 influenza A viruses and seven influenza B viruses representative of those circulating since 2006 to the Medical College of Wisconsin.

In a controlled laboratory setting, researchers there performed three separate tests for each combination of virus and rapid test.

Shively and colleagues said that the variation in performance observed between the tests could be related to differences in their design.

"Manufacturers use different antibodies in their [rapid influenza diagnostic tests] to capture nucleoprotein antigen, and this difference in antibody selection might account for some of the variation in performance," they wrote, noting that periodic evaluation of performance might "identify needed updates in antibodies."

Because of the demonstrated variability in performance, especially at low virus concentrations, a negative test result might not exclude influenza infection, they noted.

"Therefore, antiviral treatment, if indicated, should not be withheld from patients with suspected influenza because they have a negative [rapid influenza diagnostic test] result," the authors wrote.

Primary source: Morbidity and Mortality Weekly Report
Source reference: Beck E, et al "Evaluation of 11 commercially available rapid influenza diagnostic tests -- United States, 2011-2012" MMWR 2012; 61: 873-876.

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