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Based on symptoms alone, respiratory illness caused by influenza is challenging to distinguish from illness caused by other respiratory pathogens, therefore rapid diagnostic testing may be helpful for guiding antiviral treatment decisions and control activities. Most public health experts agree that rapid diagnostic testing is recommended when an institutional outbreak of influenza is suspected or if quicker test results could better determine a clinical course of action. These treatment and control decisions are influenced by the clinician’s understanding of the prevalence of influenza activity, as well as the limitations or rapid testing.
Rapid influenza tests are just one segment of tests used to detect influenza A viruses; others include viral culture, polymerase chain reaction (PCR), immunofluorescence DFA testing, and enzyme immunoassays for influenza A virus antigens, according to the Food and Drug Administration’s Center for Devices and Radiological Health (CDRH). Nasopharyngeal and nasal specimens (swab, aspirate, wash) are preferred over other upper respiratory samples, such as throat swabs, for diagnostic testing because of higher quantities of detectable virus. Specimens should be collected within the first four days of illness.
The National Center for Immunization and Respiratory Diseases (NCIRD) advises that most rapid influenza tests are designed to be greater than 70 percent sensitive for detecting influenza and greater than 90 percent specific compared with virus culture, which accounts for the fact that false negative results occur more commonly than false positive results. The CDRH notes that rapid tests have been shown to have moderate sensitivities when compared to traditional detection methods for influenza A and B virus strains, and that although rapid influenza tests cleared for use in the U.S. generally demonstrate a sensitivity of greater than 60 percent, false negatives are likely, and may vary by age and type of specimen. While specificity of cleared rapid tests is generally high (greater than 90 percent), false-positive test results occur and again may vary by age and specimen type.
The predictive values of influenza tests depend on the level of influenza activity in the community, exposure of the patient to a contagious person, susceptibility of the patient, the characteristics of the tests (sensitivity and specificity), and the adequacy of specimen collection. Inadequate or inappropriate specimens are more likely to yield false negative results. The NCIRD suggests, “The tests are most reliable when there is known influenza activity in the community and when they are performed on patients who have signs and symptoms consistent with influenza and who are within the first four days of illness. However, the symptoms and signs of influenza can vary by age and underlying medical conditions, and not all patients with influenza virus infection will manifest typical symptoms and signs of influenza.”
Rapid diagnostic tests have their limitations, the CDR cautions: “When interpreting results from any rapid influenza test, clinicians must use clinical experience, further laboratory testing, surveillance information about circulating influenza strains and the current level of influenza activity, along with an understanding of the limitations of these rapid tests.” Confirmatory testing using immunofluorescence, viral culture, or PCR should be considered when tests are negative at the beginning of the season or an outbreak, as a negative test may not rule out influenza viral infection. During peak activity when negative predictive values are lowest, false negatives are more likely. When influenza activity is low, positive results should be confirmed by immunofluorescence DFA testing, viral culture, or RT-PCR, as false positive test results are more likely. Additionally, a positive test does not rule out the possibility of co-infections with other pathogens.