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Rapid Diagnostic Tests May Miss New Swine Flu, CDC Warns

Negative test results do not rule out influenza virus infection

According to the Centers for Disease Control and Prevention (CDC), cases of influenza A (H3N2) variant (H3N2v) virus infection with the influenza A (H1N1)pdm09 M gene were detected in the U.S. during July 2011 — July 2012. In a CDC report issued on August 10, rapid influenza diagnostic tests (RIDTs) were evaluated for the detection of these viruses.

RIDTs are often used to diagnose influenza infection in clinical settings, and the recent outbreaks of H3N2v virus have highlighted the need to evaluate commercially available, widely used RIDTs for their ability to detect H3N2v viruses, the CDC explained.

The agency evaluated seven FDA-approved RIDTs with seven H3N2v viruses. Tests with BinaxNOW, Directigen, FluAlert, QuickVue, and Sofia were performed according to the procedures in the kit inserts for nasal washes or aspirates. Xpect tests were performed according to their procedure for nasal washes and swab specimens transported in liquid media. For the Veritor test, 100 mcL of diluted specimen was added directly to the reagent tube. Positive and negative controls contained in each RIDT were run before testing the viruses in the study to verify the performance of each assay lot, with the exception of FluAlert, which does not provide controls.

Only four of the seven RIDTs in this study (Directigen, Sofia, Veritor, and Xpect) detected all influenza A (H3N2)v viruses. BinaxNOW detected five of seven, and QuickVue detected three of seven. FluAlert detected only one of seven.

These findings underscore the fact that a negative RIDT result should not be considered as conclusive evidence of lack of infection with influenza A (H3N2)v, the agency cautioned. Results from RIDTs, both positive and negative, always should be interpreted in the broader context of the circulating influenza strains present in the area, the level of clinical suspicion, the severity of illness, and the risk for complications in a patient with suspected infection.

Clinicians can minimize the occurrence of false RIDT results by strictly following the manufacturer's instructions, by collecting specimens soon after the onset of influenza-like illness (ideally, within the first 72 hours), and by confirming the RIDT results by sending a specimen to a public health laboratory.

For more information, visit the CDC Web site.

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