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

What is it used for?

If you have symptoms of a respiratory infection, a test for common seasonal flu may be used to help find out if the influenza A or B virus is causing your illness. Knowing whether you have the flu is especially important if you have a high risk of becoming seriously ill from the flu. The test results can help your provider decide if medicine could help treat your illness and which one might help you.

Public health officials may also use flu tests to monitor flu and COVID-19 outbreaks in the community.

Why do I need a flu test?

If you have flu symptoms, testing can help you decide what to do next, such as starting antiviral medicine, and taking steps to lower your chances of spreading the virus to others.

Symptoms of the flu and other respiratory illnesses may include:

• Fever
• Chills
• Muscle or body aches
• Fatigue
• Headache
• Runny or stuffy nose
• Sore throat
• Cough

Even if you have flu symptoms, you may not always need a flu test. If the flu is widespread in your area, your provider may be able to diagnose you without a test.

Your provider may be more likely to order a flu test if you have symptoms and a high risk of developing a serious illness from the flu and other respiratory infections. Testing helps your provider make sure you get the right treatment to lower your risk. The risk of developing serious complications from the flu is higher for people who:

• Are age 65 and older
• Are younger than age 5 (especially those younger than 2 years old)
• Have a chronic health condition, such as asthma, blood disorders, diabetes, lung diseases, metabolic disorders, obesity, conditions that affect the heart, kidneys, liver, or nervous system, and diseases that weaken the immune system

It's important to understand your risk of serious illness from the flu before you get sick. That way you know if you need to contact your provider at the first sign of symptoms.

What happens during a flu test?

If you are tested at your provider's office or other health care setting, a fluid sample may be collected using:

• A nasopharyngeal swab test. A health care professional will insert a long swab into your nostril until it reaches your nasopharynx (the upper part of your throat). They will then rotate the swab for 10 to 15 seconds, remove it, and test that sample for signs of influenza A and B. In some cases, they may also test for influenza A subtype H5 (bird flu), and SARS-CoV-2 (Covid-19).
• A nasal aspirate or wash. A health care professional will put some saline solution (salt water) into your nose and use gentle suction to remove it.

If your provider orders an H5 bird flu test, they may use additional methods. These include:

• Conjunctival swab. Your provider may use this test if you have conjunctivitis (an eye infection). For this test, your provider will:
  • Gently pull down the lower eyelid of your infected eye.
  • Swab the inside of your lower eyelid by rotating the swab over the infected areas 2-3 times. They will not touch the surface of your eye.
• Sputum culture. This test checks a sample of your sputum (the mucus that is produced in the lungs and respiratory tract) for signs of the bird flu virus. Your provider may only order this if you have serious respiratory disease and a flu infection. For a sputum culture:
  • You may be asked to rinse your mouth out with water before the sample is taken.
  • Your provider will ask you to breathe deeply and then cough deeply into a special cup.
  • Your provider may tap you on the chest to help loosen sputum from your lungs.
  • If you have trouble coughing up enough sputum, your provider may ask you to breathe in a salty mist to help you cough more deeply.
  • If you can't cough up enough sputum after breathing in the salty mist, your provider may perform a bronchoscopy.

If you have severe respiratory disease and a flu infection, your provider may use a bronchoalveolar lavage (BAL) to collect a sample from your lungs. This is a procedure that can be done during a bronchoscopy (a procedure that's used to check for the cause of a lung problem).

If you use an at-home test, it's very important to follow all the instructions that come with your test kit. There are two types of at-home tests:

• Rapid antigen tests. You usually swab your nose to collect the fluid sample. The instructions will tell you how to complete the test, which takes under 30 minutes.
• Self-collection tests. You usually swab your nose to collect a fluid sample to send to a lab for testing, but some tests may use saliva (spit).

Will I need to do anything to prepare for the test?

You don't need any special preparations for a nasopharyngeal swab, a nasal aspirate or wash, or a conjunctival swab.

Because a sputum culture is often scheduled in the morning, your provider may encourage you to drink more water the night before. This is because drinking more water can increase the amount of sputum. You may also be asked not to eat for one or two hours before the test or take some antibiotics, as they may affect bacterial growth and your results. But don't stop taking any medicines unless your provider tells you to.

If you get a BAL and/or bronchoscopy, you may be asked to fast (not eat or drink) for up to twelve hours before the test. You may also be asked to stop taking certain medicines. But don't stop taking any medicines unless your provider tells you to.

Are there any risks to the test?

If your throat or nose is swabbed, you may feel a gagging sensation or a tickle. A nasal aspirate or wash and a conjunctival swab may feel uncomfortable. These effects are temporary.

There is no risk to providing a sputum culture.

There is very little risk to having a bronchoscopy or a BAL. Serious complications are rare, but they may include bleeding in the airways, infection, or a pneumothorax (collapse of part of your lung).

What do the results mean?

The results of your flu test tell you if signs of influenza A or B virus were found in your sample. Your results are usually described as "positive" or "negative."

A positive result means you most likely have the seasonal flu. It's important to limit your contact with others to keep from infecting them. False positives (when your test says you have the flu, but you really don't) are very uncommon. Most people who have the flu won't need antiviral medicine to get better, but your provider may prescribe it depending on your health and how long you've been sick.

To help ease your symptoms, get plenty of rest and drink lots of liquids. Over-the-counter pain relievers may help with fever and aches. But children and teens should never take aspirin for flu-like symptoms. That's because aspirin increases the risk of Reye Syndrome, a rare, but serious illness that can happen after a viral illness.

You can go back to your usual activities when these things have been true for at least 24 hours:

• Your symptoms have been getting better, and
• You haven't had a fever and you're not taking fever-reducing medicine.

A negative result means you may not have the flu. A different virus or bacteria may be causing your symptoms. But the flu can't be ruled out. That's because your results could be a false negative (when your test says you don't have the flu, but you really do.)

If you have negative test results and flu-like symptoms, ask your provider what you should do next. It's possible that your test was done when you had only a small amount of virus in your body. You may need more testing, especially if your first test was a rapid antigen flu test.

If you have questions about your results, talk with your provider.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a flu test?

To lower your risk of getting the flu:

• Avoid close contact with people who are sick.
• Wash your hands often with soap and water. Use an alcohol-based hand cleaner if soap and water aren't available.
• Avoid touching your eyes, nose, and mouth.
• Clean frequently touched surfaces, such as doorknobs and countertops.

Your best protection against serious illness from the flu is to get a yearly flu vaccine (flu shot). Other vaccines to protect you from respiratory infections include those for COVID-19, RSV and pneumonia. Talk with your provider to find out if these vaccines are right for you.