Information for Clinicians on Rapid Diagnostic Testing for Influenza

Background

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.

Reliability and Interpretation of Rapid Test Results

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 (particularly during periods of high influenza activity in the community), and specificities of rapid diagnostic tests for influenza are approximately 95-99%, 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). Guidance on interpretation of influenza testing results when influenza viruses are not known to be circulating are available.
• 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. Guidance on interpretation of influenza testing results when influenza viruses are circulatin are available

Minimize False Results

• Use rapid influenza 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 influenza diagnostic test result is positive or when the rapid influenza 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). Note that rapid influenza molecular assays that can produce results within 30 minutes with much higher sensitivity than RIDTs are available in some clinical settings.