Caution Needed When Using Rapid Flu Tests

Rapid diagnostic tests don’t always yield accurate results in detecting influenza, says a new report. (Image: Arkadi Bojaršinov/

Federal health officials have cautioned clinicians who use rapid tests to diagnose influenza that the approved tests vary considerably in their ability to detect flu in infected patients.

A report in today’s Morbidity and Mortality Weekly Report (MMWR) describes how well 11 rapid tests approved by the US Food and Drug Administration were able to detect various concentrations of 23 recently circulating influenza viruses, including 2009 influenza A(H1N1). Researchers evaluated the tests using 16 influenza A and 7 influenza B viruses representative of those circulating in the United States since 2006. They scored each test’s ability to give positive results using samples with 5 concentration levels of viral nucleoprotein antigen, which the tests target to give a positive or negative result.

Even though most of the tests detected viral antigens in samples with the highest viral concentration, 1 test didn’t uniformly detect high concentrations of influenza A viruses. Four tests detected at least 70% of B viruses in the third-highest concentration samples, and 1 test detected at least 50% of influenza A viruses in the third-highest concentration samples. The MMWR report includes charts comparing results according to rapid test brands and virus subtypes and concentration levels.

Because rapid tests yielded the most positive results from samples with the highest nucleoprotein antigen concentrations, the study authors said clinicians should collect respiratory specimens from patients within 24 to 72 hours of symptom onset, when the level of influenza virus in the body is highest. The authors added that because specimens with low antigen concentrations may be likely to produce negative test results, clinicians shouldn’t necessarily exclude an influenza diagnosis in patients with symptoms of infection.

To help clinicians judiciously use and interpret rapid influenza diagnostic tests, the Joint Commission, an organization that accredits and certifies US health care organizations and programs, offers 2 online courses and several instructional videos on a dedicated YouTube channel.

Categories: Infectious Diseases, Public Health, Viral Infections