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Delaware Health Alert Network #192

August 7, 2009 2:19 pm

Health Advisory

Delaware Division of Public Health (DPH) would like to share the following new and updated guidance from the Centers for Disease Control and Prevention (CDC) related to H1N1 influenza testing with Delaware healthcare providers.

The August 7, 2009, Morbidity and Mortality Weekly Report contains a report, “Evaluation of Rapid Influenza Diagnostic Tests for Detection of Novel Influenza A (H1N1) Virus – United States, 2009.” The article describes the performance of three commercially available rapid influenza diagnostic tests (RIDTs) compared to real-time RT-PCR for their ability to detect the novel influenza A (H1N1) virus in respiratory specimens. Overall sensitivity for the novel influenza A (H1N1) virus ranged from 40-69%, meaning these infections may not be detected by RIDTs, especially if the amount of virus in a specimen is low. For this reason, decisions regarding treatment and further testing among patients with negative results from RIDT should be based upon clinician suspicion, underlying medical conditions, severity of illness, and risk for complications in those persons suspected of having the novel influenza A (H1N1) virus infection.

Overall, the findings in this report demonstrate that these RIDTs are capable of detecting novel influenza A (H1N1) in respiratory specimens, but that many infections will be missed, especially in specimens with low viral titers. RIDTs do not distinguish among influenza A virus subtypes, and RIDT sensitivity might vary by subtype of influenza A. Therefore, when using a positive RIDT result to help determine the appropriate course of clinical treatment or other action, the result should always be interpreted in the context of currently circulating strains. Conversely, as indicated by the results of this and other studies, a negative RIDT result should not be interpreted as indicating the absence of infection. In this analysis, the sensitivity of all three assays evaluated declined as the viral titer in the specimen decreased. The amount of virus found in respiratory specimens can be affected by timing of the specimen collection; viral titers are highest in the first 3 days of illness. Other factors that can affect the amount of virus in the specimen include age (e.g., children generally shed more virus and for longer periods than adults), type of specimen collected, and transportation and storage of the specimen before testing. Testing with rRT-PCR or virus isolation should be performed if a more definitive determination of the presence of influenza virus is required.

During the 2009-2010 influenza season, the Division of Public Health Laboratory will use the real-time RT-PCR assay to test nasopharyngeal specimens from Delaware's sentinel physicians and hospitals.

For information on specimen collection, transport to the lab and receiving results, see the DPH Laboratory web site at http://www.dhss.delaware.gov/dhss/dph/lab/labs.html or call the DPHL at (302) 223-1520.

Link to August 7, 2009, Morbidity and Mortality Weekly Report:  http://www.cdc.gov/mmwr/PDF/wk/mm5830.pdf


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