Flu (Influenza) Test: MedlinePlus Medical Test

What is a flu (influenza) test?

Flu is short for influenza. It is a respiratory infection caused by viruses. Respiratory infections may affect your nose, throat, and/or lungs. Most flu tests check a sample of fluid from your nose or throat to find out if you're infected with a flu virus.

The flu usually spreads from person to person through coughing or sneezing. You can also get the flu by touching a surface that has the virus on it, and then touching your nose, eyes, or mouth. You can catch the flu at any time, but it's most common during certain times of the year, known as flu season. In the United States, flu season can begin as early as October and end as late as May.

Most people who get the flu will feel sick with muscle aches, fever, a runny or stuffy nose, and other uncomfortable symptoms that may feel like a very bad cold. They usually recover on their own in less than 2 weeks. But the flu can be serious, or even deadly, especially for children under age 5, adults age 65 and over, and people with certain chronic (long-term) illnesses, such as asthma, diabetes, or heart disease.

In some cases, your health care provider may prescribe medicine to help fight the flu virus. These medicines are called flu antivirals. They may make your symptoms milder and shorten the time you're sick by about one day. They may also help prevent serious flu complications, such as pneumonia.

Flu antivirals work best when you start taking them within 2 days after feeling sick. So early testing may be important, especially if you have a high risk of getting seriously ill from the flu. But most people get better without antiviral medicine.

Flu tests check for the most common types of seasonal flu viruses, influenza A and influenza B. Some tests are done in a health care setting, such as your provider's office, pharmacies, or outpatient clinics. You can also buy at-home tests.

There are two main types of tests for seasonal flu:

• Rapid influenza antigen tests, or rapid influenza diagnostic tests (RIDTs), are the most common flu tests. They check your fluid sample for certain proteins (antigens) found on flu viruses. These rapid tests can provide results in less than 30 minutes, and you can do them at home. But they aren't as accurate as other types of flu tests.
• Molecular flu tests are more accurate than rapid antigen tests. They check your sample for genetic material from flu viruses. There are two types of molecular flu tests:
  • Rapid molecular tests can provide results in under 30 minutes. You usually need to go to a health care setting to have these tests.
  • Molecular laboratory tests are the most accurate type of flu test, but you may have to wait up to three days for results. That's because your sample is sent to a special lab for testing. You can go to a health care setting to have your sample collected, or you can use an at-home collection kit to gather a sample and send it to the lab. A PCR (polymerase chain reaction) is a common molecular lab test.

Tests are also available to check for seasonal flu viruses along with other respiratory viruses that cause similar symptoms. These combination or "multiplex" tests use one fluid sample to check for three or more different viruses. Most often, they can be used to check for the seasonal flu and:

• COVID-19 (coronavirus disease 2019). COVID-19 is caused by the SARS-CoV-2 virus. You can get COVID-19 at any time during the year. Combination flu/COVID-19 tests are available as at-home rapid antigen tests, rapid molecular tests, and laboratory molecular tests.
• Respiratory syncytial virus infections (RSV). RSV infections are usually most common between fall and spring. Combination tests check for RSV, seasonal flu, and COVID-19 all at once. These tests are available from your provider as rapid molecular tests and molecular laboratory tests, including tests with home collection kits.

Combination tests may be especially useful during times of the year when several respiratory viruses are spreading widely. Providers treat seasonal flu, COVID-19, and RSV with different antiviral medicines. If you need antiviral medicine, a combination test helps make sure you get the right kind. It's possible to have more than one respiratory virus at the same time.

Your provider may use molecular laboratory tests to check for bird flu, also called avian influenza, if you've been exposed to this virus. Bird flu is a subgroup of uncommon influenza A viruses (called H5) that cause respiratory illness. These viruses are different from the influenza A viruses that cause seasonal flu. They usually infect birds only, but sometimes they infect other animals, including cows.

People rarely get bird flu. When they do, it's usually because they were in contact with infected birds or animals. Most bird flu infections in people have been caused by bird flu viruses called H5N1 and H7N9.

If you have flu symptoms and have been near animals that could have had bird flu, tell your provider right away and stay away from others. Your provider can order a test for H5 bird flu. The test checks a sample from your nose, throat, or eye for signs of the virus that causes bird flu.

Your provider can help you decide if you need a test that checks for more than seasonal flu. To decide which test is right for you, your provider will consider your symptoms, medical history, and how widespread different viral illnesses are in your community.

Other names: rapid flu test, influenza antigen test, rapid influenza diagnostic test, RIDT, Flu PCR, Type A and B Antigen Detection, SARS-CoV-2 & Flu A/B, Flu SC2 multiplex assay, COVID-19, Flu A+B and RSV, H5 Bird Flu test