Interpretation of Testing Results
Sensitivities and specificities of RT-PCR and other molecular assays that have been cleared by the FDA for diagnostic use are very high compared to other FDA-cleared assays that use different methods. However, even with RT-PCR, false negative results can occur due to improper or poor clinical specimen collection or from poor handling of a specimen after collection and before testing. A negative result can also occur by testing a specimen that was collected when the patient is no longer shedding detectable influenza virus. False positive results, although rare, can occur (e.g., due to lab contamination or other factors). Information on interpretation of influenza testing results is also available in the IDSA Influenza Clinical Practice Guidelinesexternal icon.
Negative result
A negative result means that there is no evidence of influenza viral RNA or nucleic acids in the respiratory specimen tested. For hospitalized patients, especially for patients with lower respiratory tract disease, if no other etiology is identified and influenza is still clinically suspected, additional specimens should be collected and tested, and antiviral treatment should be initiated or continued.
Positive result
A positive result indicates detection of influenza viral RNA or nucleic acids in the respiratory specimen tested, confirming influenza virus infection,
but does not necessarily mean infectious virus is present or that the patient is contagious.
A positive result on testing of an upper respiratory tract specimen in a person who recently received intranasal administration of live attenuated influenza virus vaccine (LAIV) may indicate detection of vaccine virus. LAIV contains influenza virus strains that undergo viral replication in respiratory tissues of lower temperature (e.g., nasal passages) than internal body temperature. Since the nasal passages are infected with live influenza virus vaccine strains during LAIV administration, sampling the nasal passages within a few days after LAIV vaccination can yield positive influenza testing results. It may be possible to detect LAIV vaccine strains up to 7 days after vaccination, and in rare situations, for longer periods.
Influenza molecular assay interpretation will depend on the individual test that is performed. For example, a negative result from an influenza molecular assay that only detects influenza A virus and the A(H1N1)pdm09 subtype does not preclude infection with influenza B virus. Clinicians can consult the package insert for detailed description of each FDA-cleared test and what the result may or may not signify.