How to diagnose influenza virus

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Select your location to view local American Lung Association events and news near you. Our service is free and we are here to help you. Diagnosing and Treating Flu. Section Menu. How Is Influenza Diagnosed? How Is Influenza Treated? Antivirals Treat Respiratory Viruses. Watch Video. Both flu viruses and the virus that causes COVID can be spread to others by people before they begin showing symptoms; by people with very mild symptoms; and by people who never experience symptoms asymptomatic people.

This means the virus that causes COVID can quickly and easily spread to a lot of people and result in continual spreading among people as time progresses. Those at highest risk include:. Most people who get flu will recover on their own in a few days to two weeks, but some people will experience severe complications , requiring hospitalization.

Some of these complications are listed above. People at higher risk of complications or who have been hospitalized for COVID or flu should receive supportive medical care to help relieve symptoms and complications. Prescription influenza antiviral drugs are FDA-approved to treat flu. People who are hospitalized with flu or who are at increased risk of complications and have flu symptoms are recommended to be treated with antiviral drugs as soon as possible after illness onset.

FDA has issued emergency use authorizations external icon EUAs to allow healthcare providers to use investigational products that are not yet approved, or that are approved for other uses, to treat patients with COVID if certain legal requirements are met external icon.

There are multiple FDA-licensed influenza vaccines produced annually to protect against the 4 flu viruses that scientists expect will circulate each year. To receive weekly email updates about Seasonal Flu, enter your email address:.

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Surveillance can also identify the predominant circulating types, influenza A virus subtypes, and strains of influenza viruses. Top of Page. A number of tests can help in the diagnosis of influenza see table.

But, tests do not need to be done on all patients with suspected influenza. For individual patients, tests are most useful when they are likely to yield clinically useful results that will help with diagnosis and treatment decisions. During a respiratory illness outbreak in a closed setting e. Diagnostic tests available for influenza include viral culture, serology, rapid antigen testing, reverse transcription polymerase chain reaction RT-PCR , immunofluorescence assays, and rapid molecular assays.

Sensitivity and specificity of any test for influenza might vary by the laboratory that performs the test, the type of test used, the time from illness onset to specimen collection, and the type of specimen tested.

Among respiratory specimens for viral isolation or rapid detection of human influenza viruses, nasopharyngeal specimens typically have higher yield than nasal or 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.

Preferred respiratory samples for influenza testing include nasopharyngeal or nasal swab, and nasal wash or aspirate, depending on which kind of test is used see table. Samples should be collected within the first days of illness. Rapid influenza diagnostic tests RIDTs provide results within approximately 15 minutes; viral culture provides results in days.

Therefore, false negative results are more common than false positive results, especially during peak influenza activity in the community. Rapid molecular assays can produce results in approximately 20 minutes with high sensitivity and specificity. Other molecular assays are increasingly becoming available and can produce results in approximately minutes with very high sensitivity and specificity.

For hospitalized patients with lower respiratory tract disease and suspected influenza, lower respiratory tract specimens should be collected and tested for influenza viruses by RT-PCR because influenza viral shedding in the lower respiratory tract may be detectable for longer periods than in the upper respiratory tract. If the patient is critically ill on invasive mechanical ventilation, and has tested negative on an upper respiratory tract specimen, including by a molecular assay, a lower respiratory tract specimen endotracheal aspirate or bronchioalveolar lavage fluid should be collected for influenza testing by RT-PCR or other molecular assays.

During outbreaks of respiratory illness when influenza is suspected, some respiratory samples should be tested by molecular assays and both rapid influenza diagnostic tests and by viral culture. During outbreaks of influenza-like illness, viral culture also can help identify other causes of illness.

During outbreaks of respiratory illness when influenza is suspected, some respiratory samples should be tested by molecular assays and viral culture. Commercial rapid influenza diagnostic tests RIDTs are antigen detection assays that can detect influenza viruses within 15 minutes with low to moderate sensitivity and high specificity.

Some tests are CLIA-waived and approved for use in any outpatient setting, whereas others must be used in a moderately complex clinical laboratory. These rapid influenza diagnostic tests differ in the types of influenza viruses they can detect and whether they can distinguish between influenza virus types. Different tests can detect 1 only influenza A viruses; 2 both influenza A and B viruses, but not distinguish between the two types; or 3 both influenza A and B viruses and distinguish between the two.