Lab Diagnostic Procedures for Influenza

There are a number of flu test kits which can help in the diagnosis of influenza. But, tests do not need to be done on all patients. For individual patients, tests are most useful when they are likely to give a doctor results that will help with diagnosis and treatment decisions. During a respiratory illness outbreak in a closed setting (e.g. hospitals, nursing home, cruise ship, boarding school, summer camp) however, testing for influenza can be very helpful in determining if influenza is the cause of the outbreak.

Preferred respiratory samples for influenza testing include nasopharyngeal or nasal swab, and nasal wash or aspirate, depending on which type of test is used. Samples should be collected within the first 4 days of illness. Rapid influenza tests provide results within 15 minutes or less; viral culture provides results in 3-10 days. Most of the rapid tests that can be done in a physician’s office are approximately 50-70% sensitive for detecting influenza and approximately greater than 90% specific. Therefore, false negative results are more common than false positive results, especially during peak influenza activity.

Diagnostic flu test kits available for influenza can include viral culture, serology, rapid antigen testing, polymerase chain reaction (PCR), and immunofluorescence assays. Sensitivity and specificity of any test for influenza might vary by the laboratory that performs the test, the type of test used, and the type of specimen tested. Among respiratory specimens for viral isolation or rapid detection, nasopharyngeal specimens are typically more effective than throat swab specimens. As with any diagnostic test, results should be evaluated in the context of other clinical and epidemiologic information available to health-care providers.

Content adapted from the CDC.

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