Information for Clinicians on Rapid Diagnostic Testing for Influenza
exclamation square light iconIs it Flu or COVID-19?
Some of the symptoms of flu and COVID-19 are similar, making it hard to tell the difference between them based on symptoms alone. Diagnostic testing can help determine if you are sick with flu or COVID-19.
Rapid influenza diagnostic tests (RIDTs) can help in the diagnosis and management of patients who present with signs and symptoms compatible with influenza. They also can be useful for helping to determine whether outbreaks of respiratory disease, such as in nursing homes and other settings, might be due to influenza. However, there are limitations of RIDTs that are important for clinicians to understand in order to interpret testing results properly. See Guidance for Clinicians on the Use of Rapid Influenza Diagnostic Tests for more information.
Proper interpretation of test results is very important for accurate clinical management of patients with suspected influenza. The reliability of rapid diagnostic tests depends largely on the conditions under which they are used. Understanding some basic considerations can minimize being misled by false-negative or false-positive results.
- Sensitivities of rapid influenza diagnostic tests are low to moderate – approximately 50-70%, which means that false negative results are common, and specificities of rapid diagnostic tests for influenza are approximately 90-95%, which means that false positive results are uncommon, when compared with viral culture or reverse transcription polymerase chain reaction (RT-PCR). FDA now requires that all RIDTs achieve 80% or higher sensitivity compared with RT-PCR.
- Some RIDTs use an analyzer device to standard the test result, and have higher sensitivities to detect influenza virus antigens than RIDTs without analyzer devices.
- False-positive results are more likely to occur when influenza prevalence in the community is low, which is generally at the beginning and end of the influenza season or periods in which influenza viruses are not circulating (e.g. summertime). View guidance on interpretation of influenza testing results when influenza viruses are not known to be circulating.
- False-negative results are more likely to occur when influenza prevalence is high in the community, which is typically at the peak of the influenza season. View guidance on interpretation of influenza testing results when influenza viruses are circulating.
- Use rapid diagnostic tests with high sensitivity and specificity.
- Collect respiratory tract specimens as early in the illness as possible, (within 3-4 days of illness onset).
- Follow manufacturer’s instructions, including handling of respiratory specimens, as described in the device package insert.
- Consider sending respiratory specimens for RT-PCR to confirm results of rapid influenza diagnostic tests especially when community influenza activity is low and the rapid diagnostic test result is positive or when the rapid diagnostic test result is negative but influenza activity in the community is high. (Contact your local or state health department for information about influenza activity).
Chartrand C, Leeflang MM, Minion J, Brewer T, Pai M. Accuracy of rapid influenza diagnostic tests: a meta-analysis. Ann Intern Med. 2012 Apr 3;156(7):500-11.
Merckx J, Wali R, Schiller I, Caya C, Gore GC, Chartrand C et al. Diagnostic Accuracy of Novel and Traditional Rapid Tests for Influenza Infection Compared With Reverse Transcriptase Polymerase Chain Reaction: A Systematic Review and Meta-analysis. Ann Intern Med. 2017 Sep 19;167(6):394-409.