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CDC: All rapid flu tests do not detect H3N2v
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Just because a rapid influenza test comes back negative for influenza A H3N2v, does not mean an influenza diagnosis can be ruled out, according to CDC researchers.
In an early release of the Morbidity and Mortality Weekly Report this week, Joseph Bresee, MD, of the Influenza Division in the National Center for Immunization and Respiratory Diseases at CDC, and colleagues reported that in an evaluation of seven FDA-cleared different rapid testing systems used to detect the H3N2, only four detected all influenza A H3N2v viruses. The four tests included, Directigen (Becton, Dickinson and Company), Sofia (Quidel Fluorescence), Veritor (Becton, Dickinson and Company) and Xpect (Oxoid).
“BinaxNOW (Alere) detected five of seven, and QuickVue (Quidel) detected three of seven. FluAlert (Thermo Fisher Scientific) detected only one of seven,” the researchers wrote. The researchers noted detection of influenza is important because there were 153 H3N2v infections reported from four states between July 12 and Aug. 9, 2012.
“Results from [rapid influenza tests], both positive and negative, always should be interpreted in the broader context of the circulating influenza strains present in the area, level of clinical suspicion, severity of illness, and risk for complications in a patient with suspected infection,” the researchers concluded.
In their report, the CDC and other researchers noted that the best way for clinicians to avoid false results is to “following the manufacturer’s instructions, collecting specimens soon after onset of influenza-like illness (ideally within the first 72 hours), and confirming results by sending a specimen to a public health laboratory.”
Disclosure: Dr. Bresee reports no relevant financial disclosures.
Perspective
Back to TopPedro A. Piedra, MD
On Aug. 17, 2012, the CDC provided an update on the H3N2v outbreaks associated with swine exposure in the United States (www.cdc.gov/flu/swineflu/influenza-variant-viruses-h3ne.htm). The number of identified infections since July 2011 had risen to 237. Most of these cases occurred in children. To date there has been limited documentation of person-to-person spread. The CDC expects these numbers will rise because of the ongoing exposure of persons to pigs that occur in the local and state fairs.
Children will soon be starting school. Schools are an excellent setting for the transmission of respiratory viruses and many of these viruses cause flu-like illnesses. There is a potential for H3N2v to spread among susceptible children in daycare centers and schools. It will be important to monitor for H3N2v in children and consider antiviral treatment with oseltamivir (Tamiflu, Genentech) or zanamivir (Relenza, GlaxoSmithKline) early after illness onset. Persons with co-morbidities, pregnant women and adults older than 65 years of age should also be considered at risk.
PCR is the most sensitive test for detecting an influenza infection; however, the PCR test is generally used in academic centers and by state or local public health laboratories. On the other hand, clinicians often use the less-sensitive rapid influenza diagnostic test for confirming an influenza infection because of ease and cost. Confirmation of an influenza infection is not required for antiviral therapy. The CDC recently evaluated seven FDA-cleared rapid influenza diagnostic tests for detecting H3N2v (MMWR. 2012;61(32):619-222). Seven different H3N2v viruses were used at varying concentrations to evaluate these test kits. Only four of the rapid influenza diagnostic tests were able to identify all the H3N2v viruses, but there were variation in the amount of viruses detected, and only two of those had the best test performance. Health care providers should consider the possibility of H3N2v illness in children with flu-like illness with a positive rapid influenza diagnostic test for influenza A during the fall, when seasonal influenza normally does not circulate. Such cases should be reported quickly to the local health department.
The CDC currently recommends that to identify H3N2v and help reduce the spread, especially in a school environment, children with flu-like illness who had recent exposure to pigs, or visited an animal fair, or had close contact with an ill person with exposure to pigs should be tested for H3N2v and stay at home until their illness resolves. The local health department should be notified and the sick child should be tested for H3N2v with the CDC Flu rRT-PCR influenza panel that is currently only available in public health laboratories.
With the influenza season approaching, changes in the H3N2v (recombination events) might improve the virus’ ability to spread within humans. It will be important to continue to monitor for human cases of influenza A/H3N2v.
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